Apparati for fabricating thin semiconductor bodies from molten material

ABSTRACT

A pressure differential can be applied across a mold sheet and a semiconductor (e.g. silicon) wafer (e.g. for solar cell) is formed thereon. Relaxation of the pressure differential can allow release of the wafer. The mold sheet may be cooler than the melt. Heat is extracted through the thickness of the forming wafer. The temperature of the solidifying body is substantially uniform across its width, resulting in low stresses and dislocation density and higher crystallographic quality. The mold sheet can allow flow of gas through it. The melt can be introduced to the sheet by: full area contact with the top of a melt; traversing a partial area contact of melt with the mold sheet, whether horizontal or vertical, or in between; and by dipping the mold into a melt. The grain size can be controlled by many means.

RELATED DOCUMENTS

Priority is hereby claimed to and this is a continuation of pending U.S.application Ser. No. 12/999,206, entitled METHODS AND APPARATI FORMAKING THIN SEMICONDUCTOR BODIES FROM MOLTEN MATERIAL, in the names ofEmanuel M. Sachs et al., filed Jun. 28, 2011, the U.S. National Phase ofPCT application PCT/US2010/026639, having an International Filing Dateof Mar. 9, 2010, which is a non-provisional of and claims priority toU.S. Provisional application Ser. No. 61/209,582, entitled METHOD ANDAPPARATUS OF MAKING THIN SEMICONDUCTOR SHEETS FROM MOLTEN MATERIAL, inthe names of Emanuel M. Sachs, Richard L. Wallace, Eerik T. Hantsoo andAdam M. Lorenz, filed on Mar. 9, 2009, and U.S. Provisional applicationSer. No. 61/224,730, entitled DIRECT KERFLESS SILICON WAFER PROCESS, inthe names of Emanuel M. Sachs, Richard L. Wallace, Eerik T. Hantsoo andAdam M. Lorenz, filed on Jul. 10, 2009, and U.S. Provisional applicationSer. No. 61/237,965, entitled KERFLESS SILICON WAFER FORMING PROCESSES,in the names of Emanuel M. Sachs, Richard L. Wallace, Adam M. Lorenz,Eerik T. Hantsoo and George David Stephen Hudelson, filed on Aug. 28,2009, each of which is hereby incorporated herein fully by reference,and priority is hereby claimed to each application mentioned above.

INTRODUCTION

Inventions disclosed herein are methods of making a sheet of silicon,which may later be used as a preform that is recrystallized to produce ahigh quality substrate for the manufacture of silicon solar cells. Otherinventions disclosed herein are methods for making a thin sheet ofsilicon that can be used for manufacture of solar cells withoutrecrystallization. Methods disclosed herein may also be used to makethin sheets from molten semiconductor materials other than silicon.

Processes are disclosed in Patent Cooperation Treaty Application No.PCT/US2008/008030, entitled, RECRYSTALLIZATION OF SEMICONDUCTOR WAFERSIN A THIN FILM CAPSULE AND RELATED PROCESSES, filed Jun. 26, 2008, inthe names of Emanuel M. Sachs, James G. Serdy, and Eerik T. Hantsoo andthe Massachusetts Institute of Technology, designating the United Statesof America, and also claiming priority to a provisional U.S.application, No. U.S. 60/937,129, filed Jun. 26, 2007. The technologydisclosed in these applications can be used to recrystallize asemiconductor to a different crystal form and is referred to herein asRecrystallization In a Capsule (RIC) technology. The RIC PCT applicationand the US provisional application is hereby incorporated fully hereinby reference. Methods disclosed herein can be used to make the startingmaterial semiconductor sheet preform, which is later recrystallizedusing RIC technology.

Certain processing schemes and architecture are disclosed in PatentCooperation Treaty Application No. PCT/US2008/002058, entitled, SOLARCELL WITH TEXTURED SURFACES Filed: Feb. 15, 2008, in the names ofEmanuel M. Sachs and James F. Bredt and the Massachusetts Institute ofTechnology, designating the United States of America, and also claimingpriority to two provisional United States applications, No. U.S.60/901,511, filed Feb. 15, 2007, and No. U.S. 61/011,933, filed Jan. 23,2008. All of the PCT application and the two US provisional applicationsare hereby incorporated fully herein by reference. The technologydisclosed in these applications is referred to herein collectively asSelf Aligned Cell (SAC) technology. Methods disclosed herein can be usedto make textured semiconductor wafers for use as a starting workpiecefor self-aligned cells disclosed in the SAC patent applications.

SUMMARY

In one embodiment of a method disclosed herein shown in FIGS. 3A, 3B, 3Cand 3D, a melt of silicon 13 is maintained and a cool sheet 15 of porousrefractory material, such as graphite, is passed over the melt so thatthe refractory material contacts the top 15 of the melt. A vacuum 17 isapplied to the top of the porous refractory sheet so as to pull theambient atmosphere through the sheet. Upon contact with the melt, twoevents take place essentially simultaneously: 1) the silicon freezes tothe cooled surface of the porous refractory sheet; and 2) the vacuumholds the silicon to the refractory sheet. The result is a thin sheet 19of silicon on a cool refractory substrate. The silicon may be releasedfrom the refractory sheet after releasing the vacuum 17. There is littleor no adhesion to the refractory sheet 5, as the refractory sheet wascool upon contact to the silicon melt 13. The method may be used to formthin sheets of other semiconductors in addition to silicon. Thefollowing discussion uses silicon as an initial example and generalizesthis later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a plenum and refractory moldsheet for use with inventions hereof;

FIG. 2 is a schematic representation of such a plenum with a refractorymold sheet having a textured surface;

FIGS. 3A, 3B, 3C and 3D are a schematic representation of method stepsof an invention hereof using a plenum such as shown in FIG. 1, with FIG.3A showing a refractory sheet contacting a melt surface; FIG. 3B showinga sheet of semiconductor formed on the mold sheet; FIG. 3C showing theplenum and mold sheet assembly removed from the melt and FIG. 3D showingthe formed semiconductor sheet released from the mold sheet upon releaseof the vacuum in the plenum;

FIGS. 4A, 4B, 4C and 4D are a schematic representation of method stepsof a semi-continuous embodiment of an invention hereof using a plenumsuch as shown in FIG. 1, with FIG. 4A showing a refractory sheetapproaching a melt surface that extends above beyond the edges of acrucible; FIG. 4B showing a sheet of semiconductor forming on the moldsheet as the mold sheet moves across the surface of the melt; FIG. 4Cshowing the plenum and mold sheet assembly removed from the melt afterhaving passed beyond it; and FIG. 4D showing the formed semiconductorsheet released from the mold sheet upon release of the vacuum in theplenum;

FIGS. 5A, 5B, 5C and 5D are a schematic representation of method stepsof another semi-continuous embodiment of an invention hereof using aplenum such as shown in FIG. 1, similar to that shown in FIGS. 4A-4D,but with the plenum passing past the melt surface such that the moldsheet surface is inclined with respect to the melt surface, with FIG. 5Ashowing a refractory sheet approaching a melt surface; FIG. 5B showing asheet of semiconductor forming on the mold sheet; FIG. 5C showing theplenum and mold sheet assembly removed from the melt; and FIG. 5Dshowing the formed semiconductor sheet released from the mold sheet;

FIG. 6 is a schematic representation of a plenum and refractory moldsheet such as shown in FIG. 1, with a backing reinforcing the refractorysheet;

FIGS. 7A, 7B, and 7C are a schematic representation of method steps ofan invention hereof using a plenum such as shown in FIG. 1 with a moldsheet that has a very large scale texture so as to provide asemiconductor sheet with such a large scale texture, with FIG. 7Ashowing a textured refractory sheet; FIG. 7B showing the texturedrefractory sheet with a conforming shell of semiconductor adheredthereto; and FIG. 7C showing the formed, textured semiconductor sheetreleased from the mold sheet.

FIGS. 8A-E show schematically, in cross-sectional view five stages of amethod and apparatus that withdraws a mold forming surface from nearlyface-to-face contact with a surface of a melt of molten material, andspins the mold surface causing molten material to accumulate at edges ofthe forming surface;

FIGS. 9A-C, show, schematically in cross-sectional view, three stages ofa method and apparatus that introduces a mold forming surface to asurface of a melt of molten material, by tilting the forming surfacedown toward the melt surface, to establish, progressively, nearlyface-to-face contact;

FIGS. 10A-E, schematically in cross-sectional view, five stages of amethod and apparatus that withdraws a mold forming surface from nearlyface-to-face contact with a surface of a melt of molten material, bytilting the forming surface away from the melt surface while moltenmaterial accumulates at an edge of the forming surface;

FIG. 11 shows, schematically, a crucible with a rim of partiallysubmerged graphite;

FIG. 12 shows, schematically, a crucible with submerged baffles forsuppression of wave motion;

FIGS. 13A and 13B show, schematically in cross-sectional view, twostages of a method and apparatus, showing detaching a meniscus of moltenmaterial from a formed semiconductor body with a meniscus controlelement that is above the free surface of the melt and that is notwetted by the molten material;

FIGS. 14A and 14B show, schematically in cross-sectional view, twostages of a method and apparatus, showing detaching a meniscus of moltenmaterial from a formed semiconductor body with a partially submergedmeniscus control element that is not wetted by the molten material;

FIG. 15 shows, schematically, a cross-sectional view of a crucible witha raised slot through which molten material can be pumped, locatedwithin a crucible, for presenting to a mold forming surface a locallyprotruding liquid surface;

FIG. 16 shows, schematically, a cross-sectional view of a crucible witha raised, moving weir over which molten material can flow, forpresenting to a moving mold surface a locally protruding liquid surface;

FIGS. 17A and 17B show, schematically, two stages of a method and anapparatus for providing a raised bump in a melt, usingmagnetohydrodynamic principles.

FIG. 18 shows, schematically, a seed crystal for growing crystals withrelatively large grain size;

FIG. 19 shows, schematically, a relatively larger, relatively weakervacuum plenum with a moving, relatively smaller, relatively strongervacuum plenum located therein;

FIG. 20 shows, schematically, a cross-sectional view of a mold sheethaving a mold surface, with blind holes facing toward the plenum, forlocalized vacuum profile control;

FIG. 21 shows, schematically, a cross-sectional view of a mold sheethaving a forming surface, with regions of different thermalconductivity, for localized vacuum profile control;

FIGS. 22A and 22B show, schematically, in cross-sectional view, twostages of a method and apparatus that provides a mold adjacent a melt ofmolten material, which mold is then moved, vertically past a meltsurface such that a body solidifies against the mold;

FIGS. 23A and 23B show, schematically in cross-sectional view, twostages of a method and apparatus that introduces a mold vertically intoa melt of molten material;

FIG. 24 shows, schematically, in cross-sectional view, a method andapparatus that provides a mold with a quantity of molten material aboveit, and a vacuum pulling from the opposite side of the mold sheet,generally below it;

FIG. 25 shows, schematically, in cross-sectional view a mechanism ofejector pins for detaching a formed solidified body from a mold surface;

FIG. 26 shows, schematically, in cross-sectional view a mechanism of astripper plate for detaching a formed solidified body from a moldsurface;

FIG. 27 shows, schematically, in cross-sectional view, a dual plenumassembly, for use aiding securing a mold sheet to a mold assembly, andalso releasing a formed wafer from a mold sheet;

FIG. 28 shows, schematically, in cross-sectional view a mold surfacethat extends beyond a region of relative vacuum application, to resultin a solidified body that is smaller in areal extent than a moldsurface;

FIG. 29 shows, schematically, in cross-sectional view a mold surfacethat has curved edges, to result in a solidified body that is moreeasily detached from a mold surface;

FIG. 30 shows, schematically, a porous mold composed of several layersof different materials and different thickness;

FIG. 31A shows, schematically, a cross-sectional view of a porous moldsheet with surface texture;

FIG. 31B shows the porous mold sheet of FIG. 31A, with a relativelylight vacuum having been applied, generating relatively small amount ofcontact area between the mold surface and the molten molding material;

FIG. 31C shows the porous mold sheet of FIG. 31A, with a relativelystrong vacuum having been applied, generating relatively larger amountof contact area between the mold surface and the molten moldingmaterial;

FIGS. 32A, 32B, 32C, 32D and 32E show, schematically, steps of a methodfor providing and using a functional material on the surface of themolten material;

FIGS. 33A, 33B, 33C, 33D, 33E, 33F, 33G and 33H show, schematically,steps of a method for growing a sacrificial wafer, growing a functionalmaterial upon the sacrificial wafer, melting away the sacrificial wafer,leaving the functional material from the wafer on the melt surface,contacting a mold forming surface to the melt at the functional materialand solidifying a formed wafer at the forming surface and removing theformed wafer from the mold surface;

FIG. 34 shows, schematically, a cross-sectional view of a substrate,typically silicon, with conical through holes;

FIG. 35 shows, schematically, a cross-sectional view of a substrate,typically silicon, for use as a mold sheet having a mold surface, withlaser cut, powder filled holes;

FIG. 36 shows, schematically, a cross-sectional view of a substrate,typically silicon, for use as a mold sheet having a mold surface, with aporous silicon internal portion, partially oxidized, and a porous SiO₂outer layer(s);

FIG. 37 shows, schematically in flow chart form steps of a method of aninvention hereof for making a microporous silicon substrate using anelectroless application of a metal seed layer and laser augmentation;and

FIG. 38 shows, schematically, a cross-sectional view of a substrate,typically silicon, for use as a mold sheet having a mold surface, with abulk silicon internal portion, pierced by oxidized porous silicon plugsand a bulk SiO2 outer layer(s).

DETAILED DESCRIPTION

General aspects of inventions disclosed herein are discussed first.Following the general aspects, detailed variations are discussed.

An aspect of an invention hereof will first be discussed in a batchimplementation, where a single semiconductor sheet is made at a time, asshown schematically with reference to FIGS. 3A-3B. In this case, thesemiconductor melt 13 may be contained in a fairly conventional crucible11 made of graphite, silica, silicon carbide, silicon nitride and othermaterials known to contain molten silicon. As shown in FIG. 1 in detaila vacuum plenum 1 is created, for example, by machining a cavity 3 intoa block of graphite. A thin sheet 5 of graphite is affixed to the bottomof the plenum 1. This sheet preferably has a fair degree of gaspermeability (having a high porosity and/or being relatively thin). Theplenum is preferably the least porous graphite available. The plenumcould also be made of non-porous ceramic. The thin sheet will bereferred to herein as the mold sheet. Vacuum suction is applied at port7. The assembly 8 of plenum 1 and mold sheet 5 is supported from aboveby a structural member (not shown). An extension of vacuum port 7 mayact as this structural member, or a separate structural member may beprovided. Referring now to FIG. 3A, the assembly 8 is brought intocontact with the surface 15 of melt 13 as in FIG. 3A. The assembly isallowed to remain in contact with the melt for a designated period oftime, perhaps on the order of 1 second. The amount of contact timebetween the assembly and the melt will vary depending on factors thatinclude, but are not limited to: the temperature of the melt, thetemperature of the mold sheet, the thickness of the mold sheet and theintended thickness of the silicon sheet to be fabricated. A siliconsheet freezes onto the mold sheet, as shown in FIG. 3B because the moldsheet 5 is colder than the freezing point of the silicon. The process isthus a transient heat transfer resulting in the silicon melt 13 beingcooled to the melting point and then heat of fusion being extracted,resulting in the buildup 19 of solid silicon on the mold sheet. Ingeneral, the mold sheet should be kept colder than the freezing point ofthe molten material. Even more generally, at least a portion of the moldsheet should be at a temperature below the freezing point, for at leasta portion of the time that the mold sheet contacts the molten material.Taking silicon as an example, the freezing/melting point is 1412° C.).Useful ranges for maintaining the mold sheet might go from roomtemperature to 1300° C., with likely range from 900° C.-1200° C., butany temperature below the freezing/melting point may be suitable,depending on the totality of the circumstances.

A principal purpose of the vacuum is to cause the silicon sheet 19 to betemporarily held against the mold sheet 5. It is helpful that thesilicon sheet 19 be easily removed from the mold sheet 5 after thesilicon sheet is formed. It is very helpful if the silicon sheet wouldsimply fall off. However, it is very important that as the silicon sheetis being formed, it adhere to the mold sheet 5. The vacuum 17accomplishes this goal. Without the vacuum, when the mold sheet isremoved after being in contact with the molten silicon 13 for theappropriate length of time, the solidified silicon 19 would likelyremain behind on the top 15 of the melt and then it would remelt.Indeed, significant adhesion is needed to remove the solidified siliconsheet 19 from the melt 13, 15 because the surface tension of the moltensilicon is holding the silicon sheet 19 down.

After the designated period of time, the assembly 8 is lifted out of themelt 13, now carrying silicon sheet 19 attached to it, as shown in FIG.3C. Finally, in FIG. 3D, the vacuum 17 is released and the formedsilicon sheet 19 can be separated from the mold sheet 5. Upon release ofthe vacuum 17 the silicon sheet 19 may simply fall off. However, somesmall amount of residual adhesion may keep the sheet from falling off.One approach is to apply positive gas pressure to the plenum of assembly8, so as to blow sheet 19 off. Another is to provide some gentlemechanical removal. Techniques for this are discussed in detail below.

The graphite mold sheet 5 must have sufficient porosity to allow forsuction sufficient to accomplish the goal of adhering to the siliconsheet 19. There are a very large variety of grades of graphite, rangingover a very large range of porosity. Thus there are many suitablechoices. Two such suitable choices are Grade UT6 and Grade 2191, bothfrom Ultra Carbon Corporation of Bay City Mich., a division of Carboneof America. Lower porosity graphites can also be used by making a moldsheet 26 thin so as to allow sufficient flow of gas through it. As shownschematically with reference to FIG. 6, if the mold sheet 26 is too thinto support itself over its full width while under vacuum, backupstructure 29 may be provided within the plenum. This structure may bemachined into the plenum in the form of ribs and posts. Alternatively, apiece of very porous graphite or other porous material may be placed inthe plenum to provide backup support. For example, extremely highporosity can be attained using ceramic filter bodies, which are known inthe art.

The porosity of the mold sheet 5 must not be so great as to allow themolten silicon 13 to enter the pores, thereby making release of thesilicon sheet 19 difficult or impossible. Two independent factorscombine to prevent silicon from entering into fine pores. First, thesurface tension of the molten silicon is too high to permit it toinfiltrate fine pores (of a non-wetting material). Second, the siliconis beginning to freeze rapidly on contact to the mold sheet and thisfreezing would be especially fast in the high surface area to volumeratio situation presented by a fine pore. The second factor is presenteven for a wetted material.

An advantage of lower porosity graphite for the mold sheet is that thegrain size of this material is smaller and the material can thereforeallow for fine finishes on the formed silicon sheet. These finishes canbe nearly mirror-like and provide for a very smooth silicon sheet.Alternatively, as shown in FIG. 2, a graphite sheet 35 may have texture9 deliberately machined into mold sheet 5, with the intent oftransferring the texture to the formed silicon sheet. This texture canthen act to trap light and also to provide the grooves needed toaccomplish manufacturing operations of the cell, such as are describedin the SAC patent applications mentioned above, such as channels formetallization, such as conductive fingers, and bus-bars. The applicationof vacuum draws the silicon melt to fill relevant texture elements, suchas grooves, channels, etc. The vacuum suction needs to overcome thesurface tension of the molten silicon to fill a texture element. Thetexture element can be modeled as a hemisphere. It follows that thesmallest hemispherical texture element that can be filled can beestimated by applying Laplace's equation, as follows:

Pressure=1 atm=2γ/r

Where γ is the surface tension of the molten silicon and r is the radiusof the hemispherical texture. For silicon with γ=0.72 N/m, r=7 microns.This is sufficiently small to allow for good light trapping, especiallysince the texture can be larger than with an etched texture (since nosilicon needs to be etched/wasted). The feature sizes of themetallization grooves are larger than those of the light trappingfeatures, and thus, the metallization grooves are easier to fill withmolten material. In fact, the light trapping texture can be done at avery large scale. The topography of the top surface can have acharacteristic feature scale that can be deeper than the thickness ofthe wafer itself. The foregoing discussion relates to using a meltsurface at approximately atmospheric pressure. Below, variations arediscussed using a melt surface at higher than atmospheric pressure,which would permit achieving smaller hemispheric texture elements thanis discussed above.

FIGS. 7A, 7B and 7C show a mold sheet 31 with large scale texture. Thescale of this texture is larger than the intended thickness of thesilicon sheet to be formed. FIG. 7B shows the mold sheet and plenumassembly with the frozen semiconductor sheet 32 in place. FIG. 7C showsthe silicon semiconductor sheet 32 after release from the mold sheet. Asshown, the amplitude of the corrugations of the formed sheet 32 is atleast three times the thickness of the sheet 32 itself.

One important issue is that when the frozen layer is lifted out of themelt, some liquid may stick to the bottom and then freeze in a way so asto make the bottom lumpy. One method to minimize this is to lift themold sheet up one edge, or corner first, thereby allowing moltenmaterial to run off the bottom of the wafer and back into the bulk ofthe melt.

The rapid disengagement of the freezing semiconductor sheet from thebulk of the liquid can be aided by lifting the mold sheet up a fewmillimeters (up to approximately 10 mm is possible without meniscusdetachment) immediately after contact with the melt. This will establisha meniscus of liquid, which will more readily drop off when the moldsheet is raised at the end of solidification. The steps of tilting theformed semi-conductor sheet to minimize excess liquid attachment arediscussed in more detail below.

Another approach to removing any residual liquid from the underside ofthe formed semiconductor sheet upon withdrawal is to rapidly spin themold sheet and attached semiconductor sheet thereby throwing theresidual molten material off to the side. This can be practiced with asquare shaped mold sheet. However, for symmetry, a round mold sheet maybe used, resulting in the casting of a disk shaped semiconductor sheetwafer. This wafer could then be laser trimmed to desired shape and sizeand the cut off pieces re-melted. The spinning of the mold sheet andwafer could commence immediately upon detachment of the meniscus, whichis effected by raising the mold sheet. Alternatively, commencement ofrotation could be the means by which meniscus detachment is effected.The liquid laterally ejected by spinning could be allowed to impact intothe side-walls of the containing crucible and drip back in to the melt.Alternatively, if only a small amount of liquid is ejected it may bedesirable to allow this liquid to be flung over the edges of thecrucible to remove them from the melt. These droplets of liquid wouldimpact a cold surface and freeze to it for later removal during plannedmaintenance. This bit of liquid will have concentrated impurities in itdue to the segregation of impurities during the solidification. Thusremoving this liquid will remove impurities from the system. The stepsof spinning the formed semiconductor sheet are discussed in more detail,and illustrated below.

It may be desirable to raise the temperature of the mold sheet up to ashigh as, for instance, 1200° C., or as high as it can be, while stillavoiding any adhesion between silicon and mold sheet. A highertemperature mold sheet will result in slower heat transfer and largergrain size in the solidified semiconductor sheet. Further, in the singlewafer batch mode now under discussion, it may be desirable for thesolidification to take as long as 5 seconds to provide for easiercontrol over the process. Further, it may be desirable to control atemperature profile across the mold sheet so as to cause thesolidification to proceed from one point or side on the mold sheet toanother, resulting in larger grains. For example, with a circular moldsheet it may be desired either to have the center hotter than theperimeter or the perimeter hotter than the center, depending on thedesired direction of grain growth. Having the initial nucleation at theperimeter may be advantageous because these small grains would then becut off during the trimming operation.

One means of effecting temperature control over the mold sheet is tohold it in position 1-2 cm above the melt so that it can gain heat andthen blow argon out through the mold sheet (via the port that will laterbe used to apply vacuum) so as to provide cooling and control thetemperature of the sheet. If the thickness of the mold sheet is variedover its extent, the flow through it will vary. Where the mold sheet isthicker, there will be less flow of cooling gas and the mold sheet willrun hotter. Another advantage of blowing argon out the mold sheet whileit is in position above the surface of the melt is that it will keepvapor such as silicon oxides, from depositing on the mold sheet.

The frozen formed semiconductor sheet may be released from the moldsheet simply by removal of vacuum. In addition, some outward flow of gascan be imposed to help separate the formed semiconductor sheet. Further,the application of pressure within the plenum so as to cause outwardflow of gas can also be used to cause the mold sheet to bow out slightlyand controllably, thereby helping to separate the formed siliconsemiconductor sheet. These and additional methods to encouragedetachment are discussed in more detail below.

In another preferred embodiment, the mold sheet is continually movedlaterally over the surface of a pool of melt. While it is possible thatthe mold sheet be a belt and that the process be continuous, it is alsopossible to be practiced with mold sheets of discrete length, forinstance on the order of 0.5-2 meters long. This mode will be referredto herein as a semi-continuous mode.

A differentiating requirement of a continuous and a semi-continuousmodes of operation is that a melt contained within and below the wallsof a crucible will not, without something else, suffice for a continuousor semi-continuous mode, because the mold sheet larger than the cruciblecannot contact the melt without interfering with the crucible walls. Onesolution is to create a bump in the melt, much as in wave soldering.This can be done by pumping the melt up through a slot and letting themelt overflow the slot and fall back down into the main pool of melt.Molten silicon can be pumped with a centrifugal pump, immersed in themelt. Alternatively, an oscillating magnetic field, such as created froma coil placed below the melt, can cause the melt to mound up due toelectromagnetic repulsion. Magneto-hydrodynamics can be used to create abump in the melt surface, by passing a current laterally in the melt andimposing a perpendicular magnetic field, to cause an upward body forceon the melt. Each of these methods is discussed in more detail below,and is illustrated with reference to figures of the drawing.

Another method for allowing the mold sheet to contact the surface of themelt in either a continuous or semi-continuous mode is shown in FIGS.4A, 4B, 4C and 4D. The melt 23 is provided in a narrow trough 21 and thetop of the melt extends over the top of the trough. The degree ofextension can be small, about, 1 to about 4 mm. The melt will remain inplace due to capillary action and will not overflow the trough. FIG. 4Ashows the mold sheet in assembly 8 prior to arrival at the melt 23. FIG.4B shows the mold sheet 5 approximately midway through its traverse overthe melt with a thickness of silicon 19 frozen to the portion of themold sheet 5, which has emerged from contact with the melt. The centerportion of the mold sheet is still in contact with the melt and heresilicon is in the process of freezing to the mold sheet. The consequenceis that the interface 21, which demarks the boundary between liquid andsolid, is inclined at an angle α relative to the bottom surface of themold sheet 5 (indicated by the extension of the upper line bounding theangle ∝). FIG. 4C shows the mold sheet 5 and vacuum assembly 8 after ithas completed its traverse, with the silicon sheet 19 still attached byvacuum 17. In FIG. 4D the vacuum 17 has been released and the siliconsheet 19 removed.

The speed of traverse of the mold sheet over the melt can be quiterapid. The contact time with the melt can vary between 0.001 and 1second or more, for instance two seconds. If the contact width is 2 cm,the corresponding traverse speeds will be 20 m/s and 2 cm/s, with speedsin the range of between about 5-20 cm/s most likely.

The heat of fusion for silicon is 1787 J/g, compared to a specific heatof 0.7 J/gK. The energy required to solidify a superheated melt isoverwhelmingly dominated by the heat of fusion, since even with 100° Ksuperheat, the sensible heat accounts for only 4% of the energy requiredto freeze. Since the process and resulting film thickness are controlledby heat transfer, the process is very tolerant to variations in melttemperature. To solidify a 200 micron thick film, the energy requiredper unit area (based on heat of fusion only) is 90 J/cm². For a highthermal conductivity substrate, the heat extraction is dominated by theheat transfer coefficient between the mold sheet and the semiconductor(e.g. silicon). As an example, typical heat transfer coefficients forrapid solidification processes are 1×10³ to 1×10⁶ W/m²K, with anexperimental value for silicon on a water cooled copper and stainlesssteel wheel determining by Uno as 4.7×10³ W/m²K. Heat flux per unit areais defined as: Q/A=h(T_(melt)−T_(substrate)).

For a 1000K temperature gradient, heat flux of 470 W/cm² would result ina solidification time of −200 ms for a thickness of 200 μm. As a checkto confirm the substrate thermal conductivity will not limit heat flow,the thermal diffusivity of graphite is approximately 0.1 cm²/s,resulting in thermal diffusion length of 1.4 mm in 0.2 sec. Assuming theenergy of solidification is taken up by a 1 mm thick surface layer ofgraphite with a specific heat of 2 J/gK, would result in a temp rise of200K in the graphite during the molding event.

In many embodiments, it will be important to keep the amount of meltavailable in the trough approximately constant during the traverse ofthe mold sheet, thus requiring that melt be admitted to the trough. Thismelt can come from a large reservoir of molten silicon that is containedin a crucible connected to the trough. The larger the reservoir, thesmaller the change in melt height in the trough during a traverse.Further control of melt height can be achieved by using a displacerpiston, for example of graphite, to compensate for the silicon withdrawnduring a traverse, the piston being moved downward.

While transient heat transfer will determine a thickness of siliconsheet that will freeze to the mold sheet during the period of contactwith the melt pool, some amount of liquid silicon may also be drawn offon the underside of the solidified silicon. To prevent this, the moldsheet may be traversed over the melt pool at an angle with respect tothe horizontal as shown schematically in FIGS. 5A, 5B, 5C and 5D. Thiswill provide a small hydrostatic head, which will drain any liquidsilicon adhered to the bottom of the solidified silicon 19, back intothe melt pool. FIGS. 5A, 5B, 5C and 5D show the same moments in theprocess sequence as was described with respect to FIGS. 4A, 4B, 4C and4D, respectively. Note that the crucible 25 in FIGS. 5A and 5B hasinclined outer walls. This inclination provides a margin of protectionagainst silicon wetting down the side wall of the crucible, especiallyduring the condition of FIG. 5B when the liquid is being drawn off theedge of the crucible.

Another important benefit of these methods is the ability to rejectimpurities into the melt and avoid having them incorporated into thegrowing silicon sheet. Most electrically harmful metallic impurities aremuch less soluble in the solid than in the liquid and hence tend to berejected at the solidification interface. In crystal growth processeswhere the liquid/solid interface moves in a controlled direction—such asis the case in these processes—this offers the opportunity to purify thefeedstock material. To be able to segregate impurities back into thebulk of the melt, the rate of advance of the liquid/solid interface mustnot be too high, or impurities get frozen into the solidified material.While the rate of formation of sheet is high in the inventions disclosedherein, the rate of advance of the solidification interface issignificantly lower, owing to the angle α of inclination of the liquidsolid interface with respect to the direction of pulling, indicated bythe arrow P, which is also parallel to the bottom surface of the moldsheet. For example, consider where the width of the trough containingmolten silicon is 2 cm and where the length of time needed for contactis 0.2 second. The pull speed is then 10 cm/sec. If the sheet beingfabricated is 200 microns thick (a typical thickness), then the rate ofadvance of the solidification interface is 200 microns in 0.2 seconds,or 1 mm/sec. This rate of advance, while high, will still allow forsegregation and attendant purification.

Segregation also impacts some intentional dopants and in standardcrystal growth methods, makes it difficult to use such dopants. Forexample, gallium is a desirable p-type dopant in silicon, but isordinarily difficult to use because segregation results in theresistivity of the grown crystal decreasing as growth continues in aningot growth or casting process. The same is true for phosphorous, ann-type dopant. However, with methods of the current inventions, dopantwill build up in the melt and reach a steady-state, which can bemaintained by proper replenishment of the melt. In this way, each waferis formed from a melt with the same level of doping and thus will itselfhave the same level of doping. Also, the formation of wafers directlyfrom the melt enables close, rapid process monitoring for bulkresistivity. Any needed change in melt dopant concentration can beeffected rapidly, along with each addition of Si feedstock.

The angle of inclination of the crystal growth interface is due to thefact that most of the heat is removed from the silicon across thethickness of the solidifying sheet (perpendicular to the direction oftraverse/pulling). As a consequence, the temperature gradients in thesolidifying sheet can be very low. This will lead to low stresses withinthe solidifying sheet and therefore low dislocation densities. Asdislocation density is a major factor degrading the electronicperformance of silicon sheet for photovoltaics, this is a majoradvantage.

The mold sheet 5 may be made of graphite, but also of a range of othermaterials including, for example, silicon carbide, silicon nitride,silica, boron carbide, pyrolitic boron nitride and alloys of theseincluding silicon oxynitride. Because the mold sheet is kept cool it isalso possible to consider materials such as aluminum oxide (which wouldotherwise lead to contamination of the melt by aluminum if contacted tothe melt while hot). Other materials, such as silicon itself, arediscussed below. In all cases, the porosity required to allow for vacuumsuction can be created by fabricating the mold sheet from powder andeither sintering or otherwise bonding the powder together in a porousbody. It is also possible to make the mold sheet of a non-porousmaterial and provide a sufficient density and multiplicity of smallholes to admit the vacuum. In general, the mold sheet must exhibitenough permeability to allow suction, but not enough to admit silicon.It should not contain transition metals or transition metallicimpurities. It should be fabricated in a thin sheet and either flat orwith texture. It must tolerate some degree of thermal stress/shock.

The foregoing has described, in general, that the forming face of themold sheet (also called a forming mold, in some cases) be at atemperature that is below the melting point of the semiconductormaterial. This must be so for a portion of the area of the forming face,for a portion of the duration of time that the forming face is incontact with the molten material, but not necessarily for the entiretime and over the entire area of the forming face. Similarly, theforegoing has described that a vacuum (or, as discussed elsewhere, apressure differential) be applied so that there is a pressuredifferential between the back, non-forming face of the forming mold, andthe molten material, so that the molten material is drawn or forcedagainst the forming mold. However, this pressure differential or vacuumneed not be applied over the entire surface area of the forming mold, oreven the entire portion that is in contact with molten material, or, forthe entire duration of time that the forming mold is in contact with themolten material.

The processes described herein rely on differential pressure appliedbetween the face of the mold sheet 5 (FIG. 1) exposed to the melt (theforming face, also called forming surface 6) and the opposite face ofthe mold sheet (the back face 4). A convenient means of applying thisdifferential pressure employs a vacuum pump to generate low pressure onthe back face 4 of the mold sheet, while using substantiallyambient-pressure gas at the melt surface 15 and thus the forming face 6of the mold sheet 5. An advantage of this embodiment is that the furnaceenclosure does not need to be sealed gas-tight nor does it need to becapable of sustaining positive pressures beyond what is required forpurge gas containment. However, in another embodiment differentialpressure between the faces of the mold sheet is generated by venting theback face 4 of the mold sheet 5 directly to atmosphere, whilemaintaining the atmosphere on the forming face of the mold sheet at apressure substantially higher than local atmospheric pressure. Anadvantage of this embodiment is that a vacuum pump is not required. Afurther advantage of this embodiment is that trans-mold sheetdifferential pressures greater than the local atmospheric pressure canbe achieved, which may confer process benefits, for example in thecreation of fine surface texture. The differential pressure may beapplied before the mold sheet contacts the surface of the moltenmaterial, or after.

When the present specification and claims use the term vacuum, it alsocorresponds to any means of developing a pressure difference between theforming face 6 and back face 4 of the mold sheet 5, regardless of theabsolute pressure at either the forming surface 6 or back 4 of the moldsheet 5. Experimentally, differential pressures ranging from 1kilopascal (kPa) to 100 kPa across the thickness of the mold sheet 5have demonstrated process viability. It should also be noted thatwhenever the present specification and the claims use the term vacuum,it is understood to mean a partial vacuum of any degree, up to andincluding a complete vacuum.

Porosity

The foregoing, and following, describe mold sheets and forming moldbodies that are porous. By porous, it is meant open-cell porosity, suchthat gas can flow through the porous body from one surface to anopposite surface. Such porous bodies may also include closed cell porousregions. It is necessary that the overall body be porous in such a wayas to allow the transmission of gas therethrough. Thus, the term porousis used herein to describe such porous bodies that allow thetransmission of gas therethrough, even though they may also includedclosed cell portions.

The growth of the forming semiconductor wafer may proceed in either acontinuous, semi-continuous or discrete mode, as discussed above. Forcontinuous growth a mold sheet must be fed over a rim or lip of amelt-containing crucible, come into contact with the melt over apredetermined distance, and then be fed out over a rim or lip of thecrucible. A flat mold sheet would require some portion of the melt toreside above the lip of the crucible. This could be accomplished by theformation of a mound or raised bump in the melt, by mechanical pumpingor magneto-hydrodynamic (MHD) forces, as discussed above and also below.Or, molten material could reside above the rim or lip of the crucible ifthe liquid meniscus was pinned at the top edge of the crucible. Thiscrucible might be in the shape of a linear trough, such as shown abovewith reference to FIGS. 4A-4D and FIGS. 5A-5D. Such a trough could befree standing, or could reside in or above another, larger crucible.This arrangement would have the advantage of retaining any melt that waslost over the edge of the trough crucible. A means of transferring meltfrom the lower, larger crucible back into the trough may be useful tominimize waste.

One issue with discrete or semi-continuous growth, as shownschematically with reference to FIGS. 8A-8E is the disposition of moltenmaterial remaining on the melt side surface of the wafer after forming.A microporous mold sheet 805 suspended from a vacuum plenum assembly 808that is dipped into the melt 813 such that the forming face of the moldsheet 805 is substantially parallel to the free melt surface 815 andthen withdrawn in the direction of the arrow W, tends to form a pendantdrop 889 (FIG. 8D). This drop interferes with subsequent waferprocessing and is generally undesirable. One method discussed above forremoving this excess liquid, and now illustrated here is to spin themold sheet 805 rapidly as indicated by the arrow S, upon withdrawal fromthe melt 813, thereby throwing the excess liquid off the rim orsegregating it to the periphery 888 of the formed wafer 819 (FIG. 8E).This method could be used with round or polygonal mold sheets 805.Excess melt can be returned to the crucible or removed from the systemas a means of impurity rejection. Excess melt localized to the rim orcorners of a spinning planar substrate can be trimmed off and returnedto the crucible. As with excess liquid, some or all of this material maybe sequestered as a means of impurity removal. This excess melt islikely to be high in rejected impurities as it will be the last tofreeze. As shown in FIG. 8E, the axis of rotation of the mold sheet 805is substantially normal to the plane of the formed wafer 819. This neednot be the case. Further, the axis of rotation is shown to be vertical,relative to a gravitational field. This also need not be the case.

Another method for dealing with a persistent drop of molten material,illustrated schematically with reference to FIGS. 9A-9C and FIGS.10A-10E is to cause a drop 1088 to form on one edge of the formed wafer19, rather than in the center. In this manner a sacrificial area of thewafer can be reserved for the purpose of accommodating the drop, whichcan be cut off of the formed wafer and fed back into the melt. Onemethod for achieving this is to tilt the entire vacuum plenum andattached forming face at an angle to the melt surface. This approachrequires the use of a vacuum connection allowing the tilt motion, andcapable of operation at liquid silicon temperatures, without release ofcontaminants that might compromise final wafer quality.

One implementation of this method is in two stages. The first stage isthe lay-in (FIG. 9A-FIG. 9C), where the forming face 6 is held at a tiltand translated down as indicated by the arrow L, until contact with themelt surface 15 is made at the lower edge 985 of the forming face. Theentire vacuum plenum with attached forming face is rotated about an axisparallel with the edge 985 of the forming face 6 in contact with themelt surface 15 in such a way as to sweep the molten material across theforming face 6. The edge 985 of the forming face that touched the meltsurface may move vertically during this event. One consequence of thisswept introduction of the molten material to the forming face 6 is toencourage lateral growth of the silicon wafer on the forming face(parallel to the plane of the forming face) so as to form an elongatedgrain structure with relatively large grains, which is desirable.Subsequent freezing onto this large-grained crystallographic templatecan occur normal to the surface of the forming face 6. Associated withthe swept introduction and lateral growth is a liquid-solid siliconinterface, which is at an angle with respect to the plane of the wafer(much as shown in FIG. 4B, but in that case in the context of a trough).

It should be noted that the same apparatus can also be used to introducethe forming face parallel to the surface of the melt so that allportions of the forming face contact the melt at the same time. In sucha case, the interface between solid and liquid silicon will besubstantially parallel to the plane of the forming sheet and of thewafer. Cases where the forming sheet is brought down parallel to themelt have the possibility of trapping small amounts of furnace ambientgas between the forming sheet and the melt, however, these small amountsof air will be removed by the vacuum which is being drawn through theforming face.

If, as shown with reference to FIGS. 10A-10E, the forming surface 6 istilted as the mold sheet 5 is removed from the melt surface 15 theliquid meniscus 1087 can be shed in a controlled manner, and anyresidual melt 1088 left at the edge 1089 of the formed wafer 19 (FIG.10E). This has the advantage of sweeping the meniscus in a linear manneracross the wafer surface, leaving behind only a very thin film of moltenmaterial. As with the spinning arrangement, the excess or segregatedmaterial may be trimmed off and returned to the melt; with some of thetrimmed material sequestered for removal of impurities from the melt andcrucible.

The surface finish, including flatness and smoothness, of the siliconwafer is determined in large part by the shedding of the liquid meniscusafter growth of the wafer. Good surface finish may be attained by thetilting motion described above but other processes may also furtherimprove surface finish. One important factor in the detachment of theliquid meniscus is the motion of the melt during the shedding of theliquid from the wafer surface, for instance by tilting, or spinning theformed wafer, as described below. A controlled, slow, smooth retractionof the wafer from the liquid leads to a smooth wafer surface.Instability of the liquid meniscus during shedding, as for examplecaused by waves in the melt, can lead to adverse surface artifacts, suchas ripples and bumps, on the surface of the wafer.

One method to improve the surface finish of the wafer is to reduce theamplitude of any wave motion of the melt, and rapidly damp any wavesthat do occur. One method for reducing wave amplitude is to use ashallow melt 13 (FIG. 3A), for example a melt depth of 5 mm or less isuseful, and even as shallow as 3 mm or 1 mm, if local particulateimpurities on the crucible bottom are not larger than about 0.5 mm. Theminimum melt depth achievable for certain non-wetting crucible 11materials, such as quartz, is dictated by the surface tension of liquidsilicon and the contact angle between liquid silicon and the cruciblematerial. As a result, to obtain very shallow melt depths in suchmaterials, a rim of wetting material may also be included to ensure fullareal coverage of the crucible. For example, as shown schematically withreference to FIG. 11 a thin (e.g. 5 mm thick) ring 1112 of graphite(which is wetting for silicon) can be used, with an outer diameter thatmatches the inner diameter of the crucible 11, and a height equal to thedesired depth of the melt 13.

Another method for reducing waves in the melt, shown schematically withreference to FIG. 12 is to use physical baffles 1214 submerged below thesurface 15 of the melt 13. These baffles impede lateral flow of liquidand quickly damps out any wave motion present in the melt.

Another means of controlling the rate of removal of melt from the formedwafer surface may be a meniscus control element. This consists of aseparate body that is moved relative to the forming face and theposition of which controls the position of detachment of the meniscusfrom the freshly formed wafer surface.

This topic discusses the stability of a meniscus attached to a formingface that is parallel to but raised from the nominal free melt surface.The equilibrium wetting angle of a liquid on a solid surface isdetermined by the surface energies of the melt and the surface. Thisangle is repeatable and is considered to be a constant for a givensystem of liquid, solid, and ambient gas. The equilibrium angle ofattachment of liquid silicon on solid silicon in an inert gas is 11degrees. Should a physical situation exist where the wetting angle isperturbed to less than 11 degrees then the meniscus attachment pointwill tend to move until equilibrium is re-established and the angle ofattachment is 11 degrees. The case of attachment of the liquid meniscusto the horizontal forming face (held parallel to the free melt surface)may be examined by using Laplace's equation, which relates the pressuredifference across a liquid-gas or liquid-liquid interface, the radii ofcurvature of that surface, and the surface energy of that interface. Thepressure across the interface can be taken as the hydrostatic pressure:

P=ρgH

(liquid density*gravity*height from free surface). The ambient gaspressure is taken as a constant in this calculation, and is consideredto be equal to the pressure in the liquid at the free melt surface.

If a linear edge of attachment is considered then there is only oneradius of curvature, and Laplace's Equation becomes

P=2γ/r.

By assuming an angle of attachment and incrementing along the surface invery small steps of swept angle the associated pressure drop and changein curvature can be solved for numerically. It was found throughiteration that the maximum stable height of the forming face above thefree melt surface was about 0.01077 m. A density value of 2530 kg/m³ anda surface tension of 0.72 N/m was assumed.

In the case of a forming face being slowly raised from the free meltsurface the following can be predicted. As long as the attachment angleof meniscus to forming face is greater than 11 degrees the system isstable and the liquid 813 remains attached to the edge of the formingface. Once the forming face is raised to the height where a furtherincrease in height would demand an angle of attachment of less than 11degrees to the flat surface of the forming face, the meniscus 887 moveslaterally until equilibrium is re-established or until the travelingmeniscus from the other edge of the forming face is met. In this casethe contact between the melt and the forming face is lost, and a largeremaining droplet 889 is left on the forming face (FIG. 8D). Very smallperturbations in the height of the melt can vary the velocity ofdetachment of the meniscus, or even reverse it temporarily. Theseperturbations can be caused by waves in the melt; which are difficult toavoid in a container of liquid (crucible) subject to agitation by theaction of the vacuum plenum and forming face.

Control of the velocity of meniscus detachment is desirable, as it hasbeen observed that the surface finish of Si sheet formed by thistechnique is highly dependent on the relative speed of withdrawal of themeniscus from the sheet surface. Techniques that better control thespeed of withdrawal of the meniscus from the surface of the freshlyformed Si sheet are well suited to control the surface finish of the Sisheet.

The above discussion of stability of meniscus attachment pertains to thecontrol of detachment velocity of the meniscus from the Si sheet. Onemeans of controlling the rate of removal of melt from the formed wafersurface may be a meniscus control element. This consists of a separatebody that is moved relative to the forming face and the position ofwhich controls the position of detachment of the meniscus from thefreshly formed wafer surface.

One configuration of meniscus detachment mechanism uses a material thatis not wetted by the melt. The material should have a wetting angle ofgreater than approximately 60 degrees with respect to the moltenmaterial in the ambient atmosphere present. FIGS. 13A and 13B show onepossible implementation of such a mechanism. In this example, a wafer 19is formed by dipping the mold sheet 5 into the melt 1313. After growthof the wafer, the mold sheet 5 is retracted above the free surface 1315of the melt 1313, to such a height that the liquid meniscus 1387 isstill attached to the melt side of the formed wafer 19 (less than0.01077 m per the above example). The meniscus control element, forinstance, consisting of a horizontal cylinder 1391 of small diameter(for example 5 mm), is translated between the formed wafer 19 and themelt 1313, in the direction indicated by the arrow M, forcing thedetachment of the liquid meniscus 1387 from the solid silicon wafer 19.This occurs due to deformation of the meniscus surface such that theattachment angle would be less than 11 degrees if the attachmentposition remained stationary. After translating the control element 1387across the full length of the wafer 19, the melt side surface 1318 ofthe wafer 19 is nearly free of liquid silicon 1387.

Another configuration of meniscus control element, shown in FIGS. 14Aand 14B, is a body 1491 of a non-wetting material, which is partiallysubmerged below the free melt surface 1415 of the melt 1413. Otherelements shown in FIGS. 14A and 14B with reference numerals that begin14, and are similar to those set in FIG. 13, which begin 13 and have thesame numerals for the least significant digits, are themselves,analogous.

The meniscus control element may be combined with vertical or tiltingmotion of the plenum and mold sheet, or may involve both vertical andlateral motion of the meniscus detaching body.

Trough

Wafer surface finish quality may be affected by the withdrawal speed ofthe meniscus from the freshly formed wafer surface. A means of tightlycontrolling this speed is desirable. Also, a staged introduction of themolten material to the mold sheet surface, in a controlled sweep mayhave benefits in the final crystallography of the formed wafer.Specifically, a lateral introduction of molten material to the formingface of the mold sheet may encourage lateral growth of all or part ofthe forming wafer—resulting in larger, elongated grain structure. Bylateral introduction, it is meant that relative motion is providedbetween the forming face and the free surface of the molten material,which relative motion has a component that is parallel to the plane ofthe free surface or tangential in the case of a curved melt freesurface. Subsequent freezing of semiconductor material, such as Si fromthe melt to this crystallographic template of elongated grain structuremay occur to reach the desired wafer thickness. One means of achievingboth the controlled introduction of melt to mold sheet surface andseparation of melt from the wafer surface is by use of the troughmentioned above, in connection with FIGS. 4A-4D and FIGS. 5A-5D and toprovide a bump, or raised portion of the melt. Means to provide such abump are discussed below. In those methods, the trough 21 (filled withmolten material) is positioned such that the melt makes contact with oneedge of the mold sheet 5, after which lateral relative motion of thetrough 21 to the mold sheet 5 is effected to sweep the molten material23 across the forming face of the mold sheet 5. Such motion might becombined with tipping of the mold sheet 5 to encourage removal of meltfrom the wafer surface, as just described. A gas jet might also be usedto force the excess melt off the wafer surface. Such a trough requires ameans of filling or replenishing, as melt would be lost to wafers beingformed, as well as possible spillage over the edge. In a discrete orsemi-continuous mode of growth, the trough can be refilled by submergingit under the surface of a melt in a larger crucible.

Pumped Raised Melt

One technique discussed generally, briefly, above to provide acontinuous process is to create a relatively raised region in the melt,referred to herein in some cases, as a bump, much as in wave soldering.Apparatus for accomplishing this is shown schematically with referenceto FIG. 15. This can be done by pumping the melt 1513 up through a slot1582 and letting the melt overflow the slot and fall back down into themain pool of melt 1513. Molten silicon can be pumped with a means 1585of continuously or discontinuously pressurizing molten silicon 1513 toflow it above the free surface 1515 of the melt, providing a locationfor attachment of molten silicon to the forming face 6 of the mold sheet5, immersed in the melt 1513. The pumping means could be a gear pump, animpeller pump or any other suitable means. In a related embodiment,pumping the molten silicon can be carried out in a non-continuous mannerusing a syringe-type displacer which mates with a silicon filledreceptacle in fluid communication with the slot 1582. This embodimenthas the advantage of allowing direct real-time control over height ofthe melt meniscus 1518 by varying the displacer position. A similarembodiment uses gas as a displacer to feed the slot non-continuouslywith molten silicon. The advantages of both these embodiments overcontinuously-pumped slots are decreased wear, decreased hardwarecomplexity, and the capability to vary meniscus height on a per-waferbasis.

Instead of relying on pumped or kinetically-forced bumps, as shownschematically with reference to FIG. 16, a locally high liquid surface1615 can instead be presented to a moving mold sheet 5 by sweeping aweir 1681 in the direction of arrow W, beneath the liquid siliconsurface 1615 along the length of the mold sheet, which is itself movedin the direction of arrow M, which has a component that is parallel tothe arrow W and which has a magnitude that exceeds the magnitude of thevelocity of the weir in the direction of the arrow W. Such a weir willtemporarily raise the liquid silicon free surface height 1685 ahead ofthe moving weir 1681. As molten silicon flows over the moving weir, themold sheet moves independently over the peak of the weir, also generallyin the same direction in the direction of arrow M, engaging the flowingliquid. Once the weir has traversed the crucible 1611 and the entirelength of the mold sheet has been passed over the edge of the liquidsurface, the completed solidified wafer 1619 can be removed, the weircan return to its starting position, and the cycle can begin again. Themold sheet may also be stationary, with the moving weir and relatedraised melt providing contact with the mold sheet, which is horizontal,slightly above the free surface. The weir can be symmetric such thatwafers can be formed in both directions.

Another method to cause a section of the melt surface to raise or lowervertically is shown schematically with reference to FIGS. 17A and 17B. Aportion of the melt 1713 is segregated in an electrically insulatingtrough 1711. Quartz could be used as a trough material. The trough 1713is itself within and in fluid communication with a larger fluidreservoir, not shown, into which the melt 1713 can flow. If electricalcontact is made to the electrically segregated region at both endsthrough two contacts 1791 a and 1791 b, a current path along thedirection of the arrow I can be localized to the liquid volume 1713defined by the interior dimension of the crucible 1711 and the depth ofmelt.

If this current is applied in a transverse magnetic field along thedirection indicated by the arrow B a body force F is created in theconfined region of melt. The direction of that body force is either upor down depending on the signs of the current I and the magnetic fieldB. If one end of the trough 1711 (shown open on the left hand side ofFIG. 17A) is allowed to communicate with the larger volume of meltresiding outside the trough (not shown) then a change in the verticalposition of the top of the liquid 1715 in the trough can be effected bythe magnitude and sign of the current, as shown by comparing the levelof the top of the liquid shown in FIGS. 17A and 17B; while stillconfining the current path and associated body force to the melt in thetrough and between the two electrodes.

The methods and apparatus described above for the lateral introductionof the mold sheet to the surface of the melt are intended to encouragelateral growth of the grains in the formed silicon wafer, leading tolong grains which exhibit enhanced electrical properties. One method tofurther increase the grain size is by seeding of the formed wafer with aseed crystal of known orientation, which can be implemented inconjunction with any of the lateral introduction methods describedabove.

One method for seeding the grown wafer is shown with reference to FIG.18. Seeding is achieved by attaching (either by vacuum or mechanically)a piece 1829 of monocrystalline silicon at the leading edge of the moldsheet 5 (Which will move relative to the melt in the direction of thearrow A (the first edge of the mold sheet to contact the melt surface).This is demonstrated schematically in FIG. 18 in a troughimplementation. After the melt 15 is introduced to the seed crystal, thewafer 1819 is formed by lateral growth of a single grain of the sameorientation as the seed crystal. After the wafer-molding is complete,the seed crystal can be cut off and re-used for the next molding event;or a new seed crystal may be used for each molding event.

It may be possible to influence the final crystallography of the formedSi sheet by initiating growth with discrete seeds, which may have aknown crystallographic orientation. These seeds might be placed on themelt side of a functional layer, and could be held onto the forming faceby vacuum. The combination of a nucleation suppressing functional layer(discussed below) with discrete crystallographic seeds may result in afinal Si sheet with large grains of a predetermined orientation. Theseseeds may consist of a strip of Si wafer arranged at the edge of theforming face, such that lateral growth occurs from this seed andpropagates across the forming face. Such a seed strip may consist of anarrow slice of <111> orientation Si wafer. Another possibility is auniformly distributed set of particles spread across the forming face.In this case, the resulting Si sheet may consist of an array of grainsof uniform size corresponding to the spacing of the initial seeds.

Lateral propagation of the solidification front along the length of aforming wafer may be advantageous in growing large grains andsimplifying the design of manufacturing hardware. While the methods andapparatus described above allow lateral propagation of and directcontrol over the solid-melt interface, methods and apparatus thatprovide lateral (in-plane) growth that is free from surface liquideffects at the solid-melt interface may also be advantageous. In allembodiments of the present inventions, wafer growth requires thermalcontact between the melt and the mold sheet, which is effected byapplying vacuum through the mold sheet. As shown schematically withreference to FIG. 19, by varying the spatial vacuum environment on theback side 1904 of the mold sheet 1905 (the side that faces away from themelt surface 1915), thermal contact in a small, strong-vacuum region1921, supplied through a strong vacuum port 1909 to a strong vacuumsource 1923, may be forced between the melt 1915 and the mold sheet1905. By varying the position of this strong-vacuum region 1921—forinstance, sweeping a line of strong vacuum down the length of a formingwafer 1919 in the direction indicated by the arrow M,—lateral growth canbe effected without the need for dynamically stable mold sheet/meltinterfaces. Such an embodiment can be realized by implementing a movingvacuum plenum 1921 behind a mold sheet 1905 that is held in continuouscontact with the melt surface 1915. The wafer 1919 solidifies at or nearthe location of the moving plenum 1921. Once the wafer 1919 is formed, aweak background level of vacuum in the remainder of the plenum 1903allows the wafer 1919 to be remain temporarily attached to the moldsheet 1905. As used herein, a strong vacuum is considered betweenapproximately 30 to approximately 80 kPa, and a weak vacuum is betweenapproximately 1 and 30 kPa.

Another method of effecting in-plane lateral propagation of thesolid-liquid interface without the need for a dynamically stablemeniscus is to spatially vary the rate of heat transfer into the moldsheet by varying the properties and geometry of the mold sheet. In oneembodiment, shown schematically with reference to FIG. 20, which is across-sectional view, the local vacuum across the area of the mold sheet2005 is varied spatially by removing material in a set of blind holes2016 in the back face 2004 of the mold sheet 2005. Regions of the moldsheet with blind holes will conduct vacuum 2017 more strongly to themelt side 2006, leading to local nucleation due to enhanced heattransfer. With the correct spacing of blind holes 2016, these locallynucleated grains will propagate laterally across regions of the moldsheet without blind holes, forming maximally large grains.

In another embodiment, shown schematically with reference to FIG. 21,inserts 2116 of material with variable thermal diffusivity are implantedat different locations throughout the mold sheet 2105, (Items shown withreference numerals preceded by 21 are analogous to items preceded by 20shown in FIG. 20, unless otherwise noted.) Regions of the mold sheetthat can conduct relatively more heat in a relatively short period oftime will tend to nucleate new grains. These grains will propagateacross regions of low heat capacity, forming large grains when they meetneighboring grains. In one embodiment, inserts of high thermaldiffusivity graphite 2116 are embedded at regular intervals within amold sheet 2105 of low thermal diffusivity silicon carbide. Grains willtend to nucleate at locations 2118 directly beneath thehigh-thermal-diffusivity inserts 2116 and expand outward from thoselocations.

In another embodiment, shown schematically with reference to FIGS. 22Aand 22B, a mold sheet 2205 may be pulled vertically, with exposure to asilicon melt 2213 from a horizontal direction, rather than the verticaldirection, as shown in the embodiments discussed above. For this method,the melt 2213 is contained in a crucible 2211 with one wall 2229 ashallower than another 2229 b, such that when the level of the surface2215 of the melt 2213 rises, it spills over the shallower edge 2229 a.The melt level could be increased by means of a displacer 2221 as shownor by any suitable method. The mold sheet 2205 is placed in closecontact with the short wall 2229 a such that rather than spilling overthe upper edge of the wall 2229 a, the molten silicon 2213 contacts themold sheet 2205, with a vacuum 2217 pulling through a port 2207 from theback side 2206 of the mold sheet 2205. A solid semiconductor sheet 2219is formed against the mold sheet 2205. The mold sheet is pulled upwardsalong the direction of the arrow M. The vacuum 2217 promotes adhesionand heat transfer into the mold sheet 2205, similarly to the casesdescribed above, of melt exposure from below the mold sheet 5.Additional relatively weak vacuum can be applied farther up on the moldsheet (not shown) to retain the silicon sheet against it as the moldsheet moves upward, until release is desired. The crucible walls 2229 a,329 b, can be non-wetting with respect to the molten material, such thata small gap between the outside of the crucible wall and the mold sheetwill not be filled by an overflowing melt because of the surface tensionof the liquid silicon. An example of a non-wetting crucible material isquartz.

An advantage of this vertical method is that the shedding of liquid fromthe forming wafer 2219 is aided by gravity. The direction of motion M ofthe mold sheet 2205 can be vertical as shown in FIG. 22A, or thedirection can be inclined. If the mold sheet 2205 inclines away from themelt 2213, the formed solid silicon sheet 2219 is supported by gravityon the upper portion of the mold sheet 2205. If the mold sheet inclinestoward the melt such that the formed solid silicon sheet would be abovethe melt, a weak vacuum can be applied on the upper portion of the moldsheet to retain the silicon sheet against the mold sheet until releaseis desired. Optimal shedding of liquid would occur when gravity actsdirectly opposite the surface tension force, which for liquid siliconagainst solid silicon is at approximately 11 degrees.

Another means of effecting a controlled, progressive, lateral attachmentof melt to mold sheet surface and detachment of melt from the formedwafer surface, is to situate the forming face such that it can beplunged into a melt vertically—with the forming face surface of a moldbody perpendicular to the free surface of the melt. This is shownschematically with reference to FIGS. 23A and 23B, with a hollowmicroporous mold body 2308 being plunged into a melt 2313 along thedirection of the arrow P, forming wafers 2319 a and 2319 b, on twoforming faces 2306 a and 2306 b, respectively of the mold body 2308,having two mold sheets 2305 a and 2305 b. Two forming surfaces, 2306 aand 2306 b are shown, which are substantially parallel and facingopposite to each other. The mold body may be one of many differentshapes, and it may have two, three, four, or more mold forming surfaces.Each of the structures 2305 a and 2305 b that back up each of theforming surfaces 2306 a and 2306 b may be considered a mold sheet asthat term is used herein. FIG. 23B shows the mold body 2308 near the endof withdrawal from the melt 2313 along the direction of the arrow W.This allows the formation of two wafers per forming event, but requiresa relatively deep crucible. Differential pressure is provided throughthe plenum 2303, for instance with a vacuum 2317 being drawn through aconduit 2307.

The foregoing discussion, before this most recent discussion of avertical dip embodiment of inventions hereof, uses the term mold sheetto refer to the element against which the molten material is molded toits final shape and surface texture. The mold sheets have been describedas generally sheet like elements, of one or more layers. This presentsection uses the term mold body, which refers to a generally threedimensional, non-sheet like element, composed of two or more mold sheetsdisposed at some geometric relation to each other. Each mold sheet has aforming surface, also referred to herein as a forming face. It is notnecessary that the individual mold elements of a mold body be sheetlike. For instance, the mold body can be entirely solid, with a porousinterior that admits of a vacuum being drawn there through, withdifferent forming faces facing outwardly away from each other around theperiphery of the mold body. As used herein, and in the claims, the termmold will be used to refer to both or either a mold sheet or a moldbody, or the individual mold sheets of a mold body, or the outerperiphery of a mold body, which embodies the shape and/or surfacetexture that is molded upon the formed wafer.

In another embodiment, shown schematically with reference to FIG. 24, acool mold sheet 2405 is pulled horizontally along the direction of arrowM, with exposure to the silicon melt 2413 that resides above the moldsheet 2405. The molten material 2413 is contained in a crucible 2411using non-wetting materials along the small gap along the bottom of thewalls 2429 a, 2429 b, which prevents the silicon melt 2413 from leaking,due to the high surface tension of liquid silicon. One wall 2429 a canbe raised to allow extraction of a solid sheet 2419, which forms againsta mold sheet 2405 with vacuum pulling from the opposite side by means ofvacuum plenum 2401, through vacuum cavity 2403 to promote adhesion andheat transfer into the mold sheet 2405.

As with the other cases, discussed above, although FIG. 24 shows themold sheet being pulled horizontally (perpendicular to a gravitationalfield) that need not be the case. The mold sheet can be pulled along aline having a horizontal component, with the molten material locatedgravitationally above it.

In all cases, the use of vacuum enables a much wider process window formold sheet temperatures and materials than would be possible withoutvacuum. Specifically, the vacuum can promote necessary adhesion for amaterial that would otherwise be non-wetting. Since non-wettingmaterials also typically exhibit low nucleation and can support greaterundercooling, this is a significant advantage in controlling theresulting grain size of the solidified silicon sheet.

As shown schematically with reference to FIG. 25, release of the formedwafer 2519 from the mold sheet 2505 can be aided by one or more smallpins 2593, which would be forced through tight-fitting holes in the moldsheet. These pins might reside within the vacuum plenum void area 2503or outside it. They should preferably contact the freshly formed wafer2519 and help to push it off the surface of the forming face 2506 afterthe wafer has been formed.

Another apparatus to achieve such release is shown schematically withreference to FIG. 26. A frame 2693 forms an annulus around the perimeterof the forming face 2606. After the wafer 2619 has been formed, theframe 2693 is pushed so that it protrudes past the plane of the formingface 2606, pushing the wafer 2619 off the forming face 2606. The framecould extend around the complete perimeter of the forming face, or onlyover some segment of it.

Another method is to apply vacuum over only a reduced area of theinterior portion of the surface 4 of the mold sheet 5. The wafer tendsto form where the vacuum is applied, so applying the vacuum over asmaller interior region may result in non-complete coverage of theforming face 6 by the wafer 19. As shown with reference to FIG. 27, thereduced vacuum area can be achieved by smaller vacuum plenum 2701 a, ora non-permeable coating 2712 on the back side of the mold sheet 2705 inthe area where vacuum is not desired. Examples of such coatings are CVDSiN (Silicon Nitride) or pyrolytic graphite. The formed wafer 2719, thendoes not extend to the sharp edges of the forming face 2706, and canrelease from a purely planar surface. This may affect both the releaseproperties of the wafer and the amount of plastic deformation the waferundergoes during the forming event. The sharp edges have been found tobe places of enhanced attachment for the formed wafer 2719, and providestrong mechanical coupling between the wafer 2719 and forming face 2706.Preventing the wafer from being mechanically continuous from edge toedge promotes relative slipping between the wafer and forming face,reducing the amount of plastic deformation and associated creation andmultiplication of dislocations and other crystallographic defects. Anysuch dislocations and defects would arise from inequalities in thecoefficient of thermal expansion of the forming face and the solidifiedwafer.

Or, with a similar, simpler embodiment, as shown schematically withreference to FIG. 28, the forming face 2806 might have extensions 2893beyond the region 2803 of the vacuum plenum, so that no molten materialis drawn to the mold sheet near to its edges, thus minimizing thestronger attachment effect that arises near edges and thus the formedwafer 2819 can be more easily released.

As shown schematically with reference to FIG. 29, the forming face 2906might also be non-planar in the region of the edges 2993, promoting agradual introduction of melt to the forming face surface 2906, andfurther reducing the opportunity for mechanical attachment of formedwafer 2919 to the forming face 2906.

Any one or more of these techniques for reducing the degree ofattachment of the formed wafer to the forming face, such as a reducedarea vacuum plenum, gas impervious layer, ejector pins, stripper plate,non-planar edge regions, etc., might be combined with any otherstructure mentioned above or hereinafter developed for a similarpurpose.

It is necessary to secure the mold sheet 5 to the plenum assembly 8.This can be done using conventional pins or another mechanicalattachment mechanism. Alternatively, as shown with reference to FIG. 27,the mold sheet 2705 can be secured to the plenum assembly 2708 by meansof a vacuum. In this case, a gas non-permeable coating 2712 is providedaround the edges of a mold sheet 2705. A secondary vacuum plenum 2701 bis applied on this non-permeable area of the mold sheet 2705 as a meansof mechanical attachment of the mold sheet to the plenum assembly 2708and when the primary plenum 2701 a is pressurized for release of thesolidified semiconductor wafer 2719. The thinner cross-hatched area 2712is a gas non-permeable coating such as pyrolytic graphite, which doesnot allow gas flow, which serves two purposes. First, it prevents vacuumfrom being applied to the outer portion of the mold sheet 2705 near theedges, so the solidified wafer 2719 is defined by the non-coated areaadjacent the cavity 2703 a, where vacuum is applied. This keeps theedges of the wafer 2719 away from the edges of the mold sheet 2705 andaids in release of the wafer 2719. The outer vacuum plenum 2701 b canalso be applied on this coated area 2712 to mechanically attach the moldsheet to the vacuum assembly 2708 when the inner plenum 2701 a is notapplying vacuum during release of the wafer 2719. One advantage of usingvacuum as a means of mechanical attachment is it does not affect thethermal mass of the mold sheet 2705, which can impact wafer thickness.Another advantage is it can provide a simpler means of automated loadingand unloading of the mold sheet 2705, which must by cycled throughattachment and detachment many times over the course of a productionday, at a rate that could range from once per minute to once every twoto three hours, given a reasonable range of duration for the forming ofeach wafer, and durability of the mold sheet.

Control of target thickness and control of thickness uniformity of theformed silicon sheet is important for use of the silicon sheets infabrication of solar cells because they can impact the strength and thethermal mass of wafers made from the formed silicon sheet wafers. Thepresent methods can be used for fabricating individual semi-conductorwafers, or for larger sheets, from which wafers can be obtained, forinstance for use in a solar cell. The present discussion will use theterm sheet, as it is more general, but it will be understood that thisthickness discussion relates also to bodies formed as wafers. Thethickness and subsequent thermal mass can be important when wafersundergo rapid thermal cycles such as metallization firing. Thin areas ofa silicon sheet can result in local weakness, breakage of wafers duringhandling and decrease yield in cell processing.

Thickness of silicon sheet formed by inventions disclosed herein isdetermined primarily by control of heat extraction from the melt 13during the molding event as discussed above. The heat flux per unit areafrom the melt is influenced by the material, thickness, and surfacetexture of the mold sheet 5, as well as the applied vacuum pressure andtemperatures of both the melt 13 and mold sheet 5.

Upon contact, the intimate thermal contact between the melt 13 and moldsheet 5 results in solidification of a sheet 19 of solid silicon, whosethickness grows based on the heat of fusion and heat flux:

V=h*(T _(melt) −T _(mold))/H _(f)

Where V is the solidification front velocity, h is the heat transfercoefficient, T is temperature, and H_(f) is the volumetric heat offusion, 4.2×10⁹ J/m³ for silicon. This simple form neglects specificheat of melt superheat, which is typically less than 5% of H_(f) asdiscussed above. The planar case with a solidification front parallel tothe forming face 6 of the mold sheet is also generally applicable to acontinuous process with the mold sheet moving across a melt surface 15and solidification front would be nearly parallel, but at a shallowangle from the mold sheet. From the literature and experimentalmeasurement, an example value of h is −5000 W/m²K, which would result insolidification front velocities of 0.1 mm/s and 1 mm/s for ΔT of 100° C.and 800° C. respectively. During the initial period after contact, thesilicon sheet thickness can be controlled by the time the mold sheet isin contact with the melt.

As heat is extracted from the melt, the mold sheet 5 will heat up basedon its own thermal mass and conductivity. For the case above with ΔT of100° C., a heat flux of 5×10⁵ W/m² through a mold sheet withconductivity k=50 W/mK will result in a temperature gradient of 10 C°/mmin the mold sheet. If a thin mold sheet is used with an insulated back,the bulk temperature of the mold sheet will rise until it is saturatedwhen T_(mold)≈T_(melt). For a 2 mm thick mold sheet with specific heatC_(p)=3.5×10⁶ J/m³K and an initial ΔT of 100° C., it would have amaximum thermal capacity of 7×10⁵ J/m², corresponding to a silicon sheetthickness of 167 μm. This provides a stabilizing mechanism for thicknesscontrol that is independent of residence time.

Similarly, mold sheets may be designed with varying thickness ofdifferent materials, including a thermally insulating layer in a stackto achieve desired silicon sheet thickness and improved uniformity thatis robust against variations in residence time. As one mold sheetexample, shown schematically in FIG. 30, a layer 3008 of a mold sheet3005 with high thermal conductivity could be used with a thickness andinitial temperature chosen such that the layer 3008 will thermallysaturate once a desired thickness of silicon sheet has formed. Bythermally saturate, it is meant that no additional semiconductormaterial can solidify, given the temperature gradient between the meltand the layer 3008. A more insulating internal layer 3007 would retardsubsequent heat flow so that the rate of solidification would besubstantially slower after this initial saturation point. The layer 3008may be nearly saturated such that solidification rate slows to less than10% of the original solidification rate. This provides for a more robustmeans of controlling thickness because it decreases sensitivity toresidence time.

Thermal diffusivity of the mold sheet material will impose an additionallimit on heat extraction if the diffusivity is not sufficient to extractthe heat through the mold sheet and the surface temperature risesinstead. Lower thermal diffusivity can favor improvements in uniformityof sheet thickness, since an increase in mold sheet surface temperaturewill decrease the solidification front velocity and decrease thesensitivity of thickness to residence time.

Upon initial contact between the mold sheet and the melt, the surface ofthe melt can be shaped to partially conform to the mold sheet. This canimpose light trapping texture as discussed above and additionally caninfluence heat transfer. The vacuum pressure applied to the back surface4 (FIG. 1) of the mold sheet 5 can be used to balance the surfacetension forces of the molten silicon and dictate the contact surfacearea of the mold sheet and subsequent heat transfer coefficient. FIG.31A shows, schematically in cross-section an example of a portion of amold sheet 3105 with a textured surface on the forming face 3106. Thedepth of grooves 3110 could be less than 1 micron for a polished surfaceor could be 20 to 50 microns on a purposefully designed texture. Underlight vacuum pressure shown in FIG. 31B, there is little driving forceto deform the melt surface 3115 and only the high points of the texturewill be in intimate contact with the melt for heat extraction. Theremaining pockets of void space between the melt and mold surface arerelatively insulating and have the affect of reducing the overall heattransfer coefficient. Once a continuous layer of solid is formed, nofurther deformation will occur. Under stronger vacuum pressure (up to 1atm or higher in a pressurized melt chamber), the melt can be forceddeeper into the grooves prior to forming a solid, effectively increasingthe contact area as shown in FIG. 31C.

As the mold sheet contacts the melt surface, which was initially at theambient pressure, the pressure changes to match the vacuum pressureapplied to the back of the mold sheet with a time constant of responseequal to:

$\tau = {\frac{M\; ɛ\; \mu}{2{\kappa\rho}}\frac{t^{2}}{RT}}$

Where M, μ, ρ, and T are the molecular weight, dynamic viscosity,density and temperature of the gas, R is the universal gas constant, andε, κ and t are the void fraction, permeability and thickness of the moldsheet. As an example, using the properties of argon at 1273° K and a 1mm thick mold sheet with permeability 1×10⁻¹⁵ m², and 5% void fraction,τ=15 ms. Mold sheet materials are available with permeability valuesthat span three orders of magnitude, so combined with thicknessselection, this time constant can be tailored to between several secondsand less than approximately one millisecond.

The time available for the liquid to deform prior to solidifying acontinuous layer is determined by the melt superheat, specific heat,undercooling prior to nucleation and heat flux. The mold sheet and meltconditions can be selected such that the time available prior tofreezing is longer than the time constant for pressure reduction anddeformation of the melt surface. Such time, prior to freezing, enablesthe liquid to better conform to the mold sheet surface (similar to thatshown in FIG. 31C) and increase contact area and subsequent heattransfer coefficient.

One means of increasing the grain size is by the use of a functionallayer between the mold sheet and the melt during the formation of thewafer. There are different sorts of functional layers, which can beprovided by different techniques, and can function in different ways.Further, these functional layers can be of many different materials.

One sort of a functional layer provides a non-nucleating interface,which allows for heat extraction from the melt and sub-cooling of theliquid, leading to lateral growth of solid crystal grains withoutnucleation of new grains, resulting in larger grain size. Another sortprovides a chemical barrier against contaminating diffusion ofimpurities from the material of the mold sheet into the formingsemiconductor wafer. Yet another functional layer may promote nucleationof grains in desired locations (seeds). Still another function such alayer can provide is to prevent adhesion of the formed body to the moldsheet.

One effective non-nucleating functional layer is a thin film of asilicon oxide, for instance, silicon dioxide, for example a 500 nm thickfilm. Differential Scanning calorimetry (DSC) experiments indicate thatsub-cooling in excess of 150° C. can be maintained between liquidsilicon and silicon dioxide layers.

Functional materials can be provided on the mold sheet forming face, oron the melt, or both. Providing a functional material on the melt isdiscussed first, followed by a discussion regarding providing thefunctional material on the mold sheet.

One approach to creating a silicon oxide functional layer is shownschematically with reference to FIGS. 32A-32E to create the functionallayer on the free surface 15 of the silicon melt 13. This can beachieved by growing a thin layer 3216 of silicon oxide directly on themelt by the introduction of an oxidizing gas species that reacts withthe silicon present on the melt surface 15 to form a thin layer ofsilicon oxide which floats on the surface of the melt (FIG. 32B). Themold sheet 5 is then dipped into the melt (FIG. 32C), forming a wafer3219 on the forming face 6 of the mold sheet 5. The mold assembly 3208is withdrawn from the melt surface 15 with a vacuum applied, lifting theformed wafer 3219 from the melt. The wafer 3219 that is removed includesa coating layer 3216 of the functional material (FIG. 32E). In thisapproach, the functional layer is re-grown between each wafer moldingevent.

Another approach to creating a non-nucleating functional layer is shownschematically with reference to FIGS. 33A-33H. This approach is to growsuch a functional layer on a solid silicon surface, which issubsequently melted back to leave only the silicon oxide. Because thekinetics of oxide growth and oxide properties differ between liquid andsolid silicon, it may prove beneficial to be able to grow the functionallayer on a solid silicon surface, rather than the free surface of thesilicon melt. In this approach, the forming face 6 of the mold sheet 5is first brought into contact with the melt surface 15, making asacrificial wafer 3318, as described above, but with small grains. Thesacrificial wafer 3318 is released from the mold sheet 5 and floats onthe surface 15 of the melt 13. An oxidizing ambient 3317 is thenintroduced to the surface of the sacrificial wafer, causing the growthof an oxide layer 3316. Either simultaneously with the oxide layergrowth, or subsequently, the sacrificial silicon wafer 3318 is meltedaway from beneath by high temperature of the melt 13, but the oxideremains for a while. Finally, the mold sheet 5 is brought into contactwith the floating oxide functional layer 3316, and a production wafer3319 is formed.

Another function that a functional layer can perform is as a chemicalbarrier to prevent, for instance, diffusion of impurities from the moldsheet to the forming semiconductor wafer. Another function that afunctional layer can perform is to prevent adhesion between mold sheetand solidified silicon sheet and thereby to facilitate release. Afunctional layer can also discourage uncontrolled grain nucleation inthe forming body. Silica can serve each of these purposes. A differentfunctional material can also promote grain nucleation at desiredlocations, if it is spatially tailored.

Another approach to creating a functional layer between the melt surfaceand the mold sheet is by creating such a functional layer on the surfaceof the mold sheet itself, for example by depositing a functional layerof silicon dioxide or silicon nitride on a graphite mold sheet. FIG. 30shows such a functional layer 30014 on a mold sheet 3005. One approachto creating such a functional layer is to directly deposit thefunctional layer in final form, for example by chemical vapor deposition(CVD), or by physical vapor deposition (PVD). Layers of hydrogen-richsilicon nitride are routinely deposited on silicon solar cells for useas anti-reflection coatings by plasma enhanced chemical vapor deposition(PECVD). Both these silicon nitride layers and electron beam depositedof SiO2 have been found effective to form a nucleation suppressingbuffer layer on the mold sheet.

Another approach is to deposit a layer of precursor material, which issubsequently converted to the final functional layer 3014. For example,a thin layer of silicon can be deposited onto the mold sheet byelectron-beam evaporation and subsequently converted to silicon dioxideby a thermal anneal in the presence of an oxidizing gas.

Another implementation of a functional layer on a forming face is theuse of a powder layer. This powder layer may consist of ceramic powders,for example silicon carbide, silicon nitride, or silicon dioxide. Thepowder layer may be single or multi-component, with powders of differingcomposition and/or particle size distribution. This powder layer may beapplied by spray and subsequent drying of a slurry.

In each of the above-described implementations, the functional layercreated can persist during the molding of multiple wafers withoutrefreshing or re-depositing the functional layer. However, it may alsobe necessary to refresh or re-deposit the functional layer between eachmolding event to obtain optimal functionality. The porosity of thedeposited, converted, or refreshed functional layer must still besufficient to allow gas passage through the plane of the functionallayer such that the vacuum or differential pressure attachment mechanismcan operate.

Thus, functional materials can be chosen from the group including butnot limited to: silicon oxide, silicon dioxide (silica), siliconcarbide, silicon nitride, silicon oxynitride, silicon oxycarbide, andboron nitride and silicon itself (as a seed).

The foregoing has discussed use of a mold sheet composed of variousmaterials, such as: graphite, silicon carbide, silicon nitride, silica,silicon oxynitride, silicon oxycarbide, boron carbide, boron nitride andalloys of these including silicon oxynitride and also, under certaincircumstances, aluminum oxide.

Silicon itself could be an excellent material to grow silicon wafers on,due to its availability in very high purity, well understood thermalproperties, and ease of growing or depositing silicon based compoundssuch as silicon dioxide, silicon nitride, silicon carbide for use as afunctional layer, discussed above. More specifically, Silicon dioxidehas been found to have excellent non-nucleating and chemical barrierproperties and can be grown on silicon. Thus, it would be desirable touse a mold sheet 5 fabricated from silicon, for certain reasons. Asignificant problem is that silicon is not permeable and thus cannottransmit a vacuum or pressure differential to the melt. However, severalmethods to make silicon permeable have been developed, and are discussedbelow.

As shown schematically with reference FIG. 34 for a schematiccross-section, laser through holes 3432 can be cut into substrates 3405of thin bulk silicon 3434 (100 to 300 um thick) with through holediameter of <3 um on at least one face of the silicon substrate, (Asused herein, the term mold sheet refers to a finished mold element,having a forming face and an obverse face. The molten material ispresented against the forming face of a mold sheet. In some cases, amold body may be used, which has several mold sheets arranged relativeto each other. The term substrate is used herein to refer to materialthat is processed to become a mold sheet.) Through-hole 3432 size andpitch can be varied to control wafer properties such as thickness andmicrostructure. After creating porosity by cutting holes, the substrate3405 can be further processed by oxidizing or coating with oxides,nitrides and carbides of silicon to form a desired outer functionallayer 3431. Other methods of processing such as reactive ion etching mayalso be used to create through holes.

If thicker substrates 3505 are desired as shown schematically withreference to FIG. 35, it may become impractical to laser cutthrough-holes with <3 um diameter. An alternative approach would be tocut large conical through holes 3532 with diameters on one face 3531 ofthe substrate 3505 of 100 to 1000 um and 10 to 100 um at the oppositeface 3533. The main body 3534 of the substrate 3505 is bulk silicon.These through holes can then be filled with powders of silicon, siliconcarbide, silicon nitride, silica or a combination of all or some ofthese silicon based compounds. The through holes can be filled by, forexample, applying a slurry of desired powders with a mix of particlesizes that is consistent with the through hole dimension and the desiredpermeability of the substrate to the face of the substrate with thelarge diameter holes. More specifically, particles close to the diameterof the small hole should be included to allow the powder mix to besecurely wedged into the hole and particles with much smaller sizeshould be used to tailor permeability. Vacuum can then be applied fromthe face 3533 of the substrate with the small holes to pack the powderparticles into the through holes. The substrate can be thermally treatedin inert or reactive atmosphere to oxidize, nitride, carbide, reactionbond, or sinter the powder mix to connect and densify the powder insidethe though holes.

Both methods using laser cut through holes described so far result inpermeable silicon with macroscopically non-uniform permeability. Thismay be desired for nucleation control. If uniform permeability isdesired, bulk silicon needs to be made permeable with pores on ananometer scale. Processes for creating thin layers of porous silicon byetching in HF:H₂O₂ with a metal catalyst layers such as silver, gold,copper have been described in the literature (for example by C. Chartierat al. in Electrochimica Acta 53 (2008) 5509-5516).

Using a Silver (Ag) assisted HF:H₂O₂ etch, several novel types ofsubstrates for growth of silicon wafers from a silicon melt can befabricated. FIG. 36 shows a thick porous silicon substrate 3605 (100 to1000 um thick) with oxidized porous silicon 3631 on the surface. Duringthe etch process two types of porous silicon are generated. Silverparticles sinking into the silicon leave behind large macro pores withhundreds of nanometer (nm) diameter. Nano-porous silicon is generated atthe surface of the sample and on the pore walls. This nano poroussilicon can be removed with an alkaline etch such as NaOH or KOH. Thenano porous silicon is much more reactive then the macro porous siliconand can be left in place if, for example, a thick SiO₂ layer is desired.After silicon etching, a cleaning step in HNO₃ is recommended to removeany residual silver from the sample. To form the SiO₂ layer, a thermaloxidation between 900° C. and 1300° C. can be performed inoxygen-containing ambient. The degree of permeability can be adjusted bythe amount of Ag deposited, the etch time, HF:H₂O₂ ratio and the bathtemperature. FIG. 36 shows an example of a substrate 3605 made by thismethod with partially oxidized porous silicon in the center 3634 andlayers of porous SiO₂ on the surface 3631.

One drawback of the methods described in the literature is that it isnot easily possible to create local areas of porous silicon whilemaintaining a smooth finish in adjacent areas. Over the time required tocompletely etch through a thick substrate, part of the silver becomesdissolved in the etch solution and etching is catalyzed over the entiresurface of the substrate, even if the silver layer was masked prior toetching in HF:H₂O₂. This problem can be solved by techniques developedby present inventors hereof. FIG. 37 shows a preferred process flow forcreating substrates with porous silicon areas.

Silicon substrates are cleaned 3761 and then plated from a silvernitrate solution 3762. By then alloying 3765 the silver with theunderlying silicon using, for example, a laser at low power settingafter deposition 3762 of the silver seed layer, regions of silversilicon alloy can be created on the sample surface. Any un-alloyedsilver can then be removed 3772 in concentrated nitric acid leavingareas of clean silicon adjacent to areas with silver silicon alloy,because the Ag—Si alloy is not etched by the nitric acid. The Ag—Sialloy will still act as a catalyst but will not contaminate the etchbath and thus bulk silicon substrates with porous silicon plugs can bemanufactured by etching the so prepared substrate in an HF:H₂O₂ solution3766. FIG. 38 shows such a substrate after oxidation 3774. Oxidizedporous silicon plugs 3832 penetrate bulk silicon 3834. The bulk siliconareas are coated with fully dense SiO₂ 3831. If a thick layer of porousSiO₂ is desired above the bulk silicon the substrate can be immersed3767 in Ag doped etch solution (about 100 ppmw of Ag is sufficient)before or after the alloyed areas have completely etched through 3766.This will result in a thick layer of nano-porous silicon at the surfaceof the sample, which will result in a thick porous SiO₂ layer afteroxidation (3834 would be porous SiO₂ instead of bulk SiO in this case).After the final silicon etch 3766 or 3767, the sample should be cleanedby rinsing 3768 in DI water. If no porous silicon is desired the samplecan then be etched in a weak caustic solution 3769 such as 1% NaOH forexample. In all cases the substrates should be cleaned in nitric acid3770 to remove residual Ag metal. With this method, 3-dimensionalstructure of bulk silicon and porous silicon can be created. Becauseporous silicon is more reactive than bulk silicon three dimensionalstructures of bulk silicon and reaction products of silicon, such as forexample SiO₂ or silicon nitride can be created. Besides suppressingnucleation with a SiO₂ layer, both control of vacuum and control of heattransfer allow further control of microstructure of wafers grown onthese substrates.

As outlined earlier a thermally grown silicon dioxide acts well as anucleation suppression layer when growing silicon. Silicon wafers withrelatively large grains (grain diameter 3 to 5 times the waferthickness) have been grown on Silicon substrates with laser throughholes or porosity generated by metal assisted etching. Microstructurewas controlled by the density and size of laser though holes. Nucleationof grains was enhanced at the site of the through holes and suppressedelsewhere, showing a high degree of control of nucleation. Nearmono-crystalline silicon was grown on oxidized porous silicon substrateswith the substrate acting a as a seed for the grown wafer.

Thus, the mold sheet can be formed of porous silicon, as just discussed,and, as outlined above, the mold sheet can be composed of: graphite,silicon carbide, silicon nitride, silica, silicon oxynitride, siliconoxycarbide, boron carbide, boron nitride, and combinations thereof,along with combinations of porous silicon, as just discussed above.

Much of the foregoing discussion has concerned a mold sheet that has atextured forming surface. However, inventions disclosed herein are alsouseful with mold sheets having an untextured, substantially smooth, andeven substantially polished mold surface.

A central aspect of some of the inventions disclosed herein is the useof a pressure differential across a mold sheet and forming a waferthereon, to control the solidification, and adhesion of thesemiconductor, typically silicon, to the mold sheet and, by laterrelaxation of the pressure differential, to allow for release of theformed wafer. This aspect greatly increases the range of parameters andmaterials available for solidifying a sheet on a substrate and alsoreduces cost. The mold sheet may be (although it need not be) cooler andeven substantially cooler than the melt, because, adhesion is created bypressure differential and is not reliant on wetting. The use of lowermold sheet temperatures also broadens the nature of available sheetmaterials. Release by reduction or even reversal of the pressuredifferential provides a rapid, economical and manufacturable method ofrelease that does not rely on the function of release coatings and theirreapplication.

Heat is extracted almost exclusively through the thickness of theforming wafer (and not along its length). Accordingly, the interfacebetween liquid and solid is substantially parallel to the mold sheetforming surface or at a relatively small acute angle to it. Thus thetemperature of the solidifying semiconductor body is substantiallyuniform across its width, resulting in low stresses and low dislocationdensity and therefore higher crystallographic quality. Segregation ofimpurities from the interface to the bulk of the melt can take place,resulting in purification of the material during growth. Dopants withlow segregation coefficients (such as gallium in silicon) can be used aseach wafer can be grown from a melt with the same dopant concentrationand therefore have the same dopant concentration.

The mold sheet must allow flow of gas through it to create and sustain apressure differential and this can be accomplished by porosity acrossthe entire area of the sheet or by concentrated porosity that isdistributed over the sheet. The mold sheet may be substantially the sizeof a single wafer or the size of multiple wafers, for example in a stripform. The introduction of the melt to the mold sheet can be implementedin a wide variety of configurations including: full area contact withthe top of a melt of material; traversing a partial area contact of meltwith the mold sheet, whether horizontal or vertical, or in between; andby dipping the mold sheet into a melt. The thickness of the solidifiedlayer can be controlled by varying the temperature of the mold sheet,the thickness of the mold sheet, the temperature of the melt and theduration of contact between mold sheet and melt. The grain size can becontrolled by the initial temperature of the mold sheet, by introducingthe mold sheet to the melt in a directional means. By directional means,it is meant, progressively, with a portion of the mold sheet contactingthe molten material first, and then additional portions contact themolten material, rather than the entire mold sheet contacting moltenmaterial all at once. The grain size can also be controlled by thenature of the material at the interface between the mold sheet and themelt surface, especially by the use of functional materials that reducethe tendency for nucleation. Removal of the formed wafer from the meltis aided by providing a mechanism to shed excess, unsolidified meltwhich would otherwise by held on by capillary action. Removal of thewafer from the mold sheet can be by reduction or reversal of thepressure differential or aided mechanically. The various methods of meltintroduction, control of solidification, removal of excess melt andremoval of wafer can be combined in any reasonable manner. To make upfor loss of material by removal of the solidified sheet, material mustbe added to the melt. This can be done by either adding solid pieces, orby adding molten material, which material was melted in a separatecontainer. The replenishment can happen between the formation of eachwafer, between the formation of batches of wafers, or on a continuousbasis. The material that is added must also contain dopant, typically inapproximately the same concentration as that desired in the solidifiedwafers. However, the level of intentional doping may be varied so as tomaintain tighter control over the doping of the solidified wafers

A useful embodiment of a method invention hereof has the followingcharacteristics. To promote large grains, the melt is introduced to themold sheet in a progressive manner, for example, by using a method oftilted lay in as described with reference to FIG. 9A and FIG. 9B. Afunctional material which reduces grain nucleation is also used, whetheron the mold sheet or on the surface of the melt. The meniscus isdetached with the aid of a meniscus control element. The mold sheet islarger than the wafer to be formed and vacuum is confined to only aportion of the mold sheet so as to facilitate release.

While portions of this description have focused on the fabrication ofsilicon sheets to be subsequently processed into solar cells, themethods disclosed herein are not limited to this application. It ispossible that the grain size and structure of the formed silicon willnot be sufficient to allow for the fabrication of solar cells directlyon the silicon sheets made by these methods. Because the solidificationis taking place across the thickness of the wafer, there is thepotential to reject impurities into the bulk of the melt, as discussedabove, and therefore to chemically refine the silicon during theprocess. Thus, it may be that while some directly fabricated sheet hasgrains too small to support the highest efficiency solar cells, it maybe possible to attain reasonable efficiency cells (perhaps 15%) atextremely low cost.

As such, the sheets of Si may be used as feedstock for theRecrystallization in Capsule (RIC) technology described at the beginningof this description.

Further, the material that is formed need not be silicon. Othersemiconductor material can be used, such as elemental semi-conductorssuch as germanium and compound semi-conductors, such as galliumarsenide.

Many techniques and mechanical aspects of the inventions have beendescribed herein. The person skilled in the art will understand thatmany of these techniques and mechanical aspects can be used with otherdisclosed techniques, even if they have not been specifically describedin use together. Any combinations, sub-combinations,sub-sub-combinations, etc., of elements disclosed herein which can beeffectively combined and used, are intended to be set forth as explicitinventions, whether claimed or not claimed. It would be impossible tospecifically set forth as an invention the many hundreds of viablecombinations that are inventive, and that are based on inventionsdisclosed herein.

Thus, inventions disclosed herein include methods, articles ofmanufacture, and manufacturing apparatus.

Method inventions disclosed herein include a method of making asemiconductor sheet preform for later re-crystallization, by contactinga cool porous mold sheet to a melt of semiconductor material, developinga pressure differential across the front and back surfaces of the moldsheet and thereby separating a thin semiconductor sheet from the melt,recrystallizing the formed preform sheet, as described in the RICapplications, and then use the semiconductor sheet as a solar cellsubstrate. Another method invention is a method of making a solar cellsubstrate using the cool mold sheet and semiconductor melt, that neednot be re-crystallized, and which may or may not have a texturedsurface. Many variations on these methods have been discussed, includingthe means by which the molten material and the mold sheet meet (dip andtilt; raised melt below the mold sheet; melt above the mold sheet; meltto the side of a vertical mold sheet; plunge a mold body into a meltvertically. Other variations relate to the method of applying a pressuredifferential; including using a full or partial vacuum with a moltensurface at atmospheric pressure, using a pressurized melt furnace,applying uniform pressure over the entire mold sheet, or applyingdifferent pressure regimes at different locations of the mold sheet.Many different methods and apparatus for removing the solidified bodyfrom the mold sheet have been discussed, including turning off thedifferential pressure across the mold sheet; applying positive pressure,mechanical pins, shaping the mold sheet to prevent adhesion, providing afunctional material that acts as a mold release, and using a dual plenumvacuum along with a gas impermeable coating on a portion of the moldsheet.

For instance, any suitable method for drawing a vacuum through a moldsheet can be used. Any semiconductor can be used as the material for thesheet preform. Various techniques can be used to prevent liquid fromadhering to the bottom of the formed sheet preform. Different functionalmaterials can be used for different purposes. Different methods ofintroducing the melt to the mold sheet can be used.

Articles of manufacture inventions disclosed herein includesemiconductor sheet preforms made according to the methods mentionedabove, either suitable for use with or without re-crystallization, withor without textured surfaces. Additional article inventions hereofinclude the various configurations of mold sheets, including those withblind holes therethrough, either filled with porous or other material,or unfilled; layered, with layers of different thicknesses and thermaldiffusivities; mold sheets that are larger than the semiconductor bodyto be solidified, and/or which have rounded edges; mold sheets havingfunctional material at the forming surface; mold sheets that have flator textured forming surfaces. Still more article inventions hereof aremold sheets composed of porous silicon, with or without macroscopicopenings therethrough, which may be filled with porous material orunfilled; and with or without an outer surface, for instance of silica,or other material.

Inventions hereof of manufacturing apparatus include arrangements oftroughs and support structure for the plenum and mold sheet, asdescribed above for a semi-continuous mode of manufacture, and theplenum and mold sheet assembly; dual plenum with sweep ability; dualplenum for mold sheet and formed wafer attachment and release.Additional apparatus inventions hereof include the various apparati toproduce a raised portion in a melt, including a moving weir, a pump andraised slot for pumping molten semiconductor up therethrough, andmagnetohydrodynamic equipment. Additional apparatus inventions hereofinclude the different arrangements for presenting molten material to acool mold sheet, including those where the molten material is below themold sheet, those where the molten material is presented to the moldsheet from above (gravitationally) and those where the molten materialis presented to a mold sheet from a side.

While particular embodiments have been shown and described, it will beunderstood by those skilled in the art that various changes andmodifications may be made without departing from the disclosure in itsbroader aspects. It is intended that all matter contained in the abovedescription and shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

This disclosure describes and discloses more than one invention. Theinventions are set forth in the claims of this and related documents,not only as filed, but also as developed during prosecution of anypatent application based on this disclosure. The inventors intend toclaim all of the various inventions to the limits permitted by the priorart, as it is subsequently determined to be. No feature described hereinis essential to each invention disclosed herein. Thus, the inventorsintend that no features described herein, but not claimed in anyparticular claim of any patent based on this disclosure, should beincorporated into any such claim.

Some assemblies of hardware, or groups of steps, are referred to hereinas an invention. However, this is not an admission that any suchassemblies or groups are necessarily patentably distinct inventions,particularly as contemplated by laws and regulations regarding thenumber of inventions that will be examined in one patent application, orunity of invention. It is intended to be a short way of saying anembodiment of an invention.

An abstract is submitted herewith. It is emphasized that this abstractis being provided to comply with the rule requiring an abstract thatwill allow examiners and other searchers to quickly ascertain thesubject matter of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims, as promised by the Patent Office's rule.

The foregoing discussion should be understood as illustrative and shouldnot be considered to be limiting in any sense. While the inventions havebeen particularly shown and described with references to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the inventions as defined by theclaims.

The corresponding structures, materials, acts and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or acts for performing the functions incombination with other claimed elements as specifically claimed.

ASPECTS OF INVENTIONS

The following aspects of inventions hereof are intended to be describedherein, and this section is to ensure that they are mentioned. They arestyled as aspects, and although they appear similar to claims, they arenot claims. However, at some point in the future, the applicants reservethe right to claim any and all of these aspects in this and any relatedapplications.

A1. A method for fabricating a semi-conductor body, the methodcomprising the steps of:

-   -   a. providing a molten semi-conductor material, having a surface;    -   b. providing a porous mold, comprising a forming surface;    -   c. providing a differential pressure regime such that pressure        at at least a portion of the forming surface is less than        pressure at the molten material surface;    -   d. contacting the forming surface to the molten material for a        contact duration such that, for at least a portion of the        contact duration:        -   i. the differential pressure regime is provided and;        -   ii. at least a portion of the forming surface is at a            temperature below a melting point of the semi-conductor            material,    -   such that a body of semi-conductor material, solidifies upon the        forming surface;    -   e. causing motion of the forming surface relative to the molten        semi-conductor material, with the solidified body upon the        forming surface; and    -   f. reducing the degree of the differential pressure regime,        thereby contributing to the solidified body detaching from the        forming surface.

A2. The method of aspect 1, the step of providing a differentialpressure regime comprising, providing at the molten material surface,atmospheric pressure, and providing at the forming surface a pressurethat is less than the atmospheric pressure.

A3. The method of aspect 2, wherein the pressure that is less than theatmospheric pressure is a partial vacuum.

A4. The method of aspect 1, the step of providing a differentialpressure regime comprising, providing at the molten material surface, apressure that exceeds atmospheric pressure, and providing atmosphericpressure at the forming face.

A5. The method of aspect 1, the mold comprising a single formingsurface, which contacts a surface of the molten semi-conductor material.

A6. The method of aspect 1, the mold comprising a plurality of surfaces,which are angled relative to each other, the step of contactingcomprising dipping the mold into the molten material, such that themolten material contacts the plurality of surfaces.

A7. The method of aspect 6, the mold comprising orthogonal surfaces.

A8. The method of aspect 1, further comprising, before the step ofcontacting the forming surface to the molten material, the step oftilting the forming surface relative to the surface of the moltenmaterial, so that only a portion of the forming surface makes initialcontact with the surface of the molten material.

A9. The method of aspect 1, further comprising, after the step ofcontacting the forming surface to the molten material, the step oftilting the forming surface relative to a gravitational field, so thatmolten material flows off from the forming surface.

A10. The method of aspect 1, further comprising, during or after thestep of causing motion of the forming surface relative to the moltenmaterial, the step of spinning the forming surface about an axis havinga component that is perpendicular to the forming surface, so that moltenmaterial flows off from the forming surface.

A11. The method of aspect 1, further comprising, after the step ofcausing motion of the forming surface relative to the molten material,the step of providing a pressure regime, such that pressure between theforming surface and the solidified semi-conductor body is greater thanpressure at a free face of the solidified semi-conductor body, whichfree face, faces away from the forming surface.

A12. The method of aspect 1, further comprising the step of providing afunctional material between the forming surface and the molten material,the functional material chosen to contribute to a function chosen fromthe group of:

-   -   a. suppressing nucleation of grain growth;    -   b. preventing passage of impurities from the mold to the        solidified semiconductor body;    -   c. enhancing release of the solidified semiconductor body from        the forming surface; and    -   d. encouraging nucleation of grain growth at specific locations        of the solidified semiconductor body.

A13. The method of aspect 1, further comprising the steps of:

-   -   a. before the step of causing relative motion, the step of        providing on the solidified semi-conductor material, which is        designated a sacrificial solidified body, functional material        comprising an oxide;    -   b. melting the solidified sacrificial solidified semi-conductor        body, thereby leaving a functional body upon the molten        material; and    -   c. contacting the forming surface to the functional body upon        the molten material for a second contact duration such that, for        at least a portion of the second contact duration:        -   i. a differential pressure regime is provided and;        -   ii. at least a portion of the forming surface is at a            temperature below a melting point of the semi-conductor            material,    -   such that a body of semi-conductor material, having a free face,        solidifies upon the forming surface.

A14. The method of aspect 13, further comprising, before the step ofcontacting the forming surface to the functional body, the step ofcausing relative motion of the forming surface relative to thefunctional body, such that they are spaced apart for a time.

A15. The method of aspect 13, wherein the step of contacting the formingsurface to the functional body, comprises maintaining the formingsurface in contact with the functional body during the step of meltingthe sacrificial body.

A16. The method of aspect 12, the step of providing a functionalmaterial comprising providing the functional material to the formingsurface.

A17. The method of aspect 12, the step of providing a functionalmaterial comprising providing the functional material to the surface ofthe molten material.

A18. The method of aspect 17, the molten material comprising silicon,the step of providing functional material comprising providing a body ofsolidified functional material to the surface of the molten silicon, andenriching the atmosphere at the surface of the molten silicon in oxygen,thereby giving rise to a body of SiO2 upon the surface of the moltensilicon, further comprising the step of contacting the forming surfaceto the body of SiO2.

A19. The method of aspect 12, the functional material being selectedfrom the group consisting of: silicon oxide, silicon dioxide (silica),silicon carbide, silicon nitride, silicon, silicon oxynitride, siliconoxycarbide, and boron nitride.

A20. The method of aspect 1, further comprising providing a preferentialnucleation agent at an interface between the forming surface and themolten material, before the step of contacting the forming surface tothe molten material.

A21. The method of aspect 8, the functional material being selected fromthe group consisting of: silicon and silicon dioxide.

A22. The method of aspect 1, further comprising the step of applying ameniscus control element to detach adhering molten material from thesolidified body.

A23. The method of aspect 22, the meniscus control element comprising abody that is substantially non-wetted by the molten material, having awetting angle of greater than about 60 degrees with respect to themolten material and the ambient atmosphere.

A24. The method of aspect 22, the meniscus control element comprising arod.

A25. The method of aspect 1, the forming surface comprising asubstantially untextured surface.

A26. The method of aspect 1, the forming surface comprising a texturedsurface.

A27. The method of aspect 26, the textured surface comprising shapesthat correspond to light trapping texture to be formed in the solidifiedsemiconductor material.

A28. The method of aspect 26, the textured surface comprising shapesthat correspond to electrode locating texture to be formed in thesolidified semiconductor material.

A29. The method of aspect 26, the textured surface having acharacteristic feature scale, and the solidified semi-conductor bodyhaving a thickness that is less than the characteristic feature scale.

A30. The method of aspect 1, the porous forming body comprising amaterial selected from the group consisting of: graphite, siliconcarbide, silicon nitride, silica, silicon oxynitride silicon oxycarbide,and boron nitride.

A31. The method of aspect 1, the porous mold comprising a body ofsintered powder.

A32. The method of aspect 1, the porous mold comprising a graphite body.

A33. The method of aspect 1, the porous mold comprising a body oforiginally solid silicon that has been processed to be porous.

A34. The method of aspect 1, the porous mold comprising an originallysolid silicon body with holes that have been formed therethrough.

A35. The method of aspect 34, the holes comprising conical holes.

A36. The method of aspect 34, further comprising, in the holes throughthe solid body, porous material.

A37. The method of aspect 1, the porous mold comprising a body of poroussilicon.

A38. The method of aspect 37, the porous silicon having been formed bydepositing a metal seed layer upon a surface of a silicon body and thenetching the seeded silicon body.

A39. The method of aspect 33, the porous mold further comprising atleast one outer surface layer of a silicon oxide.

A40. The method of aspect 1, further comprising the step of controllingnucleation of grain growth at selected locations of the forming surface.

A41. The method of aspect 1, the step of controlling nucleation beingselected from the group consisting of:

-   -   a. providing a mold with a spatially varied thickness;    -   b. providing a pressure differential that varies spatially with        respect to the forming surface;    -   c. providing a mold with spatially varied thermal insulation;    -   d. providing a forming surface with a spatially varied texture;    -   e. providing a mold with a spatially varied thermal diffusivity;    -   f. providing an area-specific temperature profile at the forming        face; and    -   g. providing a crystal seed at a location upon the forming        surface that first contacts the molten material.

A42. The method of aspect 1, further comprising the step of controllingdirectionality of growth of grains at selected locations of the formingsurface.

A43. The method of aspect 42, the step of controlling directionality ofgrowth of grains selected from the group consisting of:

-   -   a. providing a mold with a spatially varied thickness;    -   b. providing a pressure differential that varies spatially with        respect to the forming surface;    -   c. providing a mold with spatially varied thermal insulation;    -   d. providing a forming surface with a spatially varied texture;    -   e. providing a mold with a spatially varied thermal diffusivity;    -   f. providing area-specific temperature profile at the forming        face; and    -   g. providing a crystal seed at a location upon the forming        surface that first contacts the molten material.

A44. The method of aspect 1, the mold comprising a plenum.

A45. The method of aspect 44, the mold comprising a mold sheet, theplenum comprising a structure designed to reinforce the mold sheetagainst any excessive pressure.

A46. The method of aspect 44, the plenum comprising a compound plenum,having at least two chambers, wherein the step of providing adifferential pressure regime comprises providing two differentdifferential pressure regimes, such that pressure at at least twodifferent portions of the forming face is less than that of anatmosphere at the molten material surface, and further comprising duringthe contacting step, the step of moving one chamber relative to theother, to change the relative location of the two different pressureregimes.

A47. The method of aspect 1, further wherein:

-   -   a. the step of providing molten material comprises providing        molten material in a container, the container having at least        one wall, such that a meniscus of the molten material exists        with a convex curvature facing away from the container having an        uppermost part that is above the wall; and    -   b. the step of contacting the forming surface to the molten        material comprises passing the forming face against the convex        meniscus.

A48. The method of aspect 47, the step of causing relative motioncomprising causing substantially linear relative motion between theforming surface and the molten material.

A49. The method of aspect 48, the step of causing relative motioncomprising causing relative motion that is substantially perpendicularto a local gravitational field.

A50. The method of aspect 48, the step of causing relative motioncomprising causing relative motion that has a component that is alignedwith a local gravitational field.

A51. The method of aspect 1, the step of providing a differentialpressure regime comprising:

-   -   a. providing a first differential pressure adjacent a first        region of the mold surface; and    -   b. providing a second, different differential pressure at a        plurality of discrete locations of the mold surface.

A52. The method of aspect 1, further comprising the step of suppressingoscillatory motion of the surface of the molten material.

A53. The method of aspect 1, the step of providing molten materialcomprising providing molten material in a vessel, such that the moltenmaterial has a depth of less than approximately five mm, and preferablyless than approximately three mm.

A54. The method of aspect 1, the step of reducing the degree ofdifferential pressure regime comprising reversing the direction ofdifferential pressure, such that a force is applied to the solidifiedmaterial directed away from the forming surface.

A55. The method of aspect 1, further comprising the step of providingthe forming surface of the mold and the surface of the molten materialeach at uniform initial temperatures across their spatial extent.

A56. The method of aspect 1, the step of contacting comprisingcontacting the forming surface to the surface of the molten material,such that each portion of the forming surface contacts the moltenmaterial for approximately the same duration.

A57. The method of aspect 56, the step of contacting comprisingproviding a progressive relative sweep of the forming surface relativeto the surface of the molten material.

A58. The method of aspect 1, further wherein the mold has a limited heatcapacity, such that the temperature of the forming surface risessubstantially to a temperature approximately equal to that of the moltenmaterial, such that thereafter, no additional molten materialsolidifies.

A59. The method of aspect 1, further comprising the step of decreasingthe heat transfer coefficient between the mold and the solidified bodyby reducing contact area between the initial solidified layer and mold.

A60. The method of aspect 59, the step of controlling the heat transfercoefficient comprising adjusting the magnitude of the differentialpressure regime.

A61. The method of aspect 59, wherein the step of providing a moldcomprises providing a mold with properties of gas permeability, voidfraction and thickness selected to control changes in the magnitude ofthe differential pressure regime over time in conjunction withsuperheating of the molten material to define a contact area for theheat transfer coefficient.

A62. The method of aspect 1, wherein the formed wafer has an impuritylevel that is lower than the impurity level in the molten material

A63. The method of aspect 62, where the lower level of impurity isaccomplished through the action of segregation and advance of asolidification front is kept slow enough to allow for segregation totake place.

A64. The method of aspect 1, further comprising a dopant with a lowsegregation coefficient.

A65. The method of aspect 64, further comprising the step of addinggallium, indium, phosphorous, or arsenic and the molten material issilicon.

A66. The method of aspect 64, further comprising the step of addingmaterial to replenish the melt, which material has a concentration ofdopant approximately equal to that desired in a final wafer.

Having described the inventions disclosed herein, what is claimed is:

1. An apparatus for fabricating a semi-conductor body, the apparatuscomprising: a. means for retaining a molten semi-conductor material,having a surface; b. a porous mold, comprising a forming surface; c.means for providing a differential pressure regime such that pressure atat least a portion of the forming surface is less than pressure at themolten material surface; d. means for contacting the forming surface tothe molten material for a contact duration such that, for at least aportion of the contact duration, the differential pressure regime isprovided such that a body of semi-conductor material, solidifies uponthe forming surface; and means for detaching the solidified body fromthe forming surface.
 2. The apparatus of claim 1, further comprisingmeans for providing a functional material between the forming surfaceand the molten material.
 3. The apparatus of claim 1, further comprisingmeans for controlling nucleation of grain growth.
 4. The apparatus ofclaim 1, further comprising means for controlling directionality ofgrain growth.
 5. The apparatus of claim 1, further comprising means formaintaining at least a portion of the forming face at a temperaturebelow a melting point of the semiconductor material.
 6. The apparatus ofclaim 1, further wherein the mold has a spatially varied thickness. 7.The apparatus of claim 1, further comprising means for providing apressure differential that varies spatially with respect to the formingsurface.
 8. The apparatus of claim 1, further wherein the mold hasspatially varied thermal insulation.
 9. The apparatus of claim 1, theforming surface comprising a spatially varied texture.
 10. The apparatusof claim 1, further wherein the mold has a spatially varied thermaldiffusivity.
 11. The apparatus of claim 1, further comprising means forproviding area-specific temperature profile at the forming face.
 12. Theapparatus of claim 1, further comprising means for providing a crystalseed at a location upon the forming surface that would first contact themolten material.
 13. The apparatus of claim 1, further comprising meansfor suppressing oscillatory motion of the surface of the moltenmaterial.
 14. The apparatus of claim 1, further comprising at least onebaffle in the molten material.